Across:
8 Most of the investigation (“case” without “e” = CAS-) in (-IN-) Oxford (-O) (CASINO) is synonymous with a number of risks (CASINO),
9 Dance (= anagram indicator) troop (troop = PROTO-) company (-CO-) left (-L) (PROTOCOL) a number of conventions (PROTOCOL),
10 Cut out (EDIT) bad (= anagram indicator) diet (diet = EDIT),
11 “Dirty stuff on the field (MUD-) at school (-S-) is not going to go away” (-LINGER) (MUD-SLINGER) – is the originator of a smear campaign (MUD-SLINGER),
12 Emigrant loses it (‘emigrant’ loses ‘it’ = emgran = GERMAN), in a manner of speaking, in Munich (GERMAN),
14 Threatening (MENACING) criminal (= anagram indicator) came in (came in = MENACI-) with poor result (no good/no grade = -NG) (MENACING),
15 Sounds like (= homophone indicator) Pius (“Pius” = “pious” = HOLY), a resident of the eternal city (ROMAN) and superpower (EMPIRE) (HOLY ROMAN EMPIRE), united some Christian countries in the past (HOLY ROMAN EMPIRE),
18 Important records (KEY NOTES) opening doors (KEY) for the musicians (NOTES) (KEY NOTES),
20 Some desperado ran German (some ‘desperado ran German’ = ORANGE) order (ORANGE order),
22 Italicised (italicised = IDEALISTIC) type (= anagram indicator) is fanciful (IDEALISTIC),
24 Immersed in developing Leningrad (immersed in “developing Leningrad’” = GLEN) valley (GLEN),
25 Fog (M-IST) covering old (-O-), old city (-UR-) on one side of the bridge (one side of the ‘bridge’ = -E) (MOISTURE) with rising water perhaps (MOISTURE),
26 Economising (SAVING) to rescue (save) most of the characters (e) (‘save’ without ‘e’ = SAV-) in (-IN-) Government (-G) (SAVING).
Down:
1 Manage (HANDLE) one of those on the door of nightclub, for instance (HANDLE),
2 General ill feeling troubles leaders (‘general ill feeling troubles’ leaders/first letters = GIFT) for the present (GIFT),
3 Knowledge of French, for instance (COMMAND- of French), is only the start of it (‘only’ the start of it = -O) (COMMANDO) in the Foreign Legion (COMMANDO),
4 Type of chip (SPUD) from bottle in Christmas pudding (bottle in ‘Christmas pudding’ = SPUD),
5 Grand (G-) office (-O-) for lawyer (Bachelor of Law = -BL-) – trendy (-IN) (GOBLIN) for doing a bit of devilling perhaps (GOBLIN),
6 Plenty of good things (CORNUCOPIA) for ring (-O-) giving run (run = -RNU-) around (= anagram indicator) to law enforcement officer (-COP-) in spy organisation (C-IA) (CORNUCOPIA),
7 Head of state (GOVERNOR) from Oregon (Oregon = GO-ERNO-) troubled over sign (Victory sign = -V- sign) from Russian leader (‘Russian’ leader = -R) (GOVERNOR),
13 Spread (MAYONNAISE) in Ireland (MAYO-) Sienna (Sienna = -NNAISE) whipped up (= anagram indicator) (MAYONNAISE),
14 Battered (= anagram indicator) team (team = MEAT-) players finish (‘players’ finish = -S) (MEATS) with different cuts (MEATS),
16 Is only slightly open to anyone making meringues (OVEN DOOR) in the kitchen (OVEN) and that will only work if you have 1 down (DOOR will only work with ‘handle’) (OVEN DOOR),
17 Cast out (EXORCISE) of Free Spirits (EXORCISE),
19 Stumble (TRIP UP) in square (T- square) going on tear (tear = -RIP UP) (TRIP UP),
21 Olive and Laurel (types of GREENS) take data from Great Danes (take ‘data’ from ‘Great Danes’ = grenes = GREENS),
23 Tightrope walker goes around (‘tightrope walker’ goes around = TIER) storey (TIER),
24 Grant (GIVE) ran out of vinegar (‘ran’ out of ‘vinegar’ = vieg = GIVE).