Across:
8 Distinguished scientist (SAVANT) designed (=anagram indicator) Sat Nav (satnav = SAVANT),
9 Rhapsodies (RAPTURES) and music (RAP-) tempo (-T-) for sure (sure = -URES) hit (= anagram indicator) (RAPTURES),
10 Stare (GAWK) from miser leaving wigmakers (from ‘miser’ leaving ‘wigmakers’ = wgak = GAWK),
11 They’re in the house (in The House/Dáil = FIANNA FAIL) for the party (FIANNA FAIL party),
12 It’s impracticable to drop plectra (‘impracticable’ to drop ‘plectra’ = imicab = IAMBIC), as it’ll create stress for Yeats (IAMBIC verse),
14 Most of 14 down (most of ‘pipes’ = PIPE) and 2 down (“rack” = RACK) (PIPE RACK) is for row of potential smokers (PIPE RACK),
15 Car (AUTO-) on grid (-GRAPH) beside Orion (Orion, the HUNTER and giant) (AUTOGRAPH HUNTER) is in need of a name everyone recognises on paper (AUTOGRAPH HUNTER),
18 Gangsters (HOODLUMS) in hold, Sumo (hold Sumo = HOODLUMS) wrestling (= anagram indicator),
20 Speculation (THEORY) by several banks? The same old story! (several banks of ‘the same old story’ = THEORY),
22 Mixing (= anagram indicator) damp cement (camp cement = DECAMPMENT) in Split, out of the blue (split/leave suddenly or secretly = DECAMPMENT),
24 Hot stuff running down hill (LAVA) discovered in Shangri-La valley (LAVA discovered in ‘Shangri-la valley’),
25 Writer (PEN-) gets stuck into what the chef is providing (-KNIFE) (PENKNIFE) for the scout’s knapsack (PENKNIFE),
26 Break (= anagram indicator) finger (finger = FRINGE) in Trim (fabric trim/frill = FRINGE).
Down:
1 Costa Rican neighbour (PANAMA), a (-A) worker (man = -NAM-) from press agency (Associated Press = PA-), retires completely (= reverse all letters) (PANAMA),
2 Scratching the head (t), of course (‘track’ without ‘t’ = RACK) of one of the lambs (RACK of lamb),
3 Commercial vehicle (STAFF CAR) for major employer (STAFF CAR for major employer/ army),
4 Operatic song (ARIA) from familiar film is missing (from “familiar” “film” is missing = aiar = ARIA),
5 In retrospect (= reversal indicator), double meaning (pun = -N UP) for cyclists’ performance-enhancing drug? (Erythropoietin = EPO = OPE-) (OPEN UP) That’s what those doing the policing said! (OPEN UP),
6 Foreign party (-ANC-) starts (= positional indicator) to pay the price (SUFFER-) – English (-E) (SUFFERANCE) will have to tolerate it and offer it up perhaps (SUFFERANCE),
7 Run-of-the-mill (MEDIOCRE) doctor (MEDI-C-) admits it’s going around (-O-) and going about (-RE) (MEDIOCRE),
13 Relatives (BLOOD) on both sides of The Liffey, for instance (BANKS) (BLOOD BANKS), bound to give the bleeding hearts what they need (BLOOD BANKS),
14 Churchwarden’s (churchwardens are clay PIPES) pastries (PI-ES) – soft (soft/piano = -P-) at the centre (PIPES),
16 Still under wraps (UNOPENED) in organisation (United Nations = UN-) with old (-O-) pair of writers (pen + ed = -PENED) (UNOPENED),
17 Make friends (HIT IT OFF) with crowd-pleaser (HIT) and one (I-) posh bloke (toff = -T OFF) (HIT IT OFF),
19 Outstanding (UNPAID) organisation (UN-) is rewarded for its work (-PAID) (UNPAID),
21 Replace (= anagram indicator) a grave (a grave = RAVAGE) in ruin (RAVAGE),
23 Apparent in bone density (apparent in ‘bone density’ = EDEN) one associates with the first gardeners (EDEN),
24 Set down (LAID down) area (-A-) in covering (L-ID) (LAID).