* These 11 solutions (not defined) can be paired with English counties. Wordplay in each of the remaining 17 clues contains a superfluous letter; these letters generate a cryptic hint (7,2,2,6): RELEASE IN AN ISLAND = UNTIE in COS = CO<UNTIE>S.
** Eleven of those 17 clues contain a superfluous word, each defining one of the 11 thematic pairs: 11 ac – journalist; 12 ac – cricketer; 15 ac – author; 26 ac – food; 28 ac – presenter; 1 dn – actress; 2 dn – actor; 5 dn – horse; 9 dn – cheese; 18 dn – serviceman; 23 dn -- superhero
Across
7* Clark (Kent) (superhero) CL(e/A)RK
8 leviathan R LEVI<A>T(e) + HA[R]N
10* Adrian (Durham) (presenter) D(irector) in NAIRA (rev)
11** notarial E NOT[E] + A<AIR>L(iberal)
12** saboteur L A/BO[L]T in SURE (anag)
13 reef E REEF[E](r)
15** confess A C(hapter)/ON + FE[A]S(t)S [T = time]
17* Malcolm (Devon) (cricketer) MAL<COL>M
20* Joey (Essex) (actor) JO(t) + EY(e)
22 Aberdeen S DANE’[S] BEER (anag)
25* dumpling (Norfolk) (food) DUMP + LING
26** confit E C(hef)/ON[E]/FIT
27 octagonal I G(ames) in A/LOCAT[I]ON (anag)
28** amend N [N]AME/N(ew)/D(egree)
Down
1** gladiator A G[A]LA + ROT/AID (rev)
2** trail off N TRAI[N]/L(atin)/OFF(on holiday)
3* Leonard (Cheshire) (serviceman) LEO(n)/NARD
4* Victoria (Derbyshire) (journalist) VICTOR + IA (Iowa)
5** starve I STAR(leading)/V[I]E
6* Sarah (Lancashire) (actress) HARAS(s) (rev)
9** once S (t)O(w)N’(s) [S(e)]C(r)E(t)
14* Blue Vinny (Dorset) (cheese) blew (hom) + VINNY (Jones)
16 sky pilot L SKY(broadcaster) + P<IL[L]>OT
18** codename A C[A]NOE MADE (anag)
19* Maugham (Somerset) (author) A/UGH in (i)MAM
21 orphan N HARP O[N] N(early) (anag)
23** etch D ET (film) + CH(a)[D] [nostalgia]
24* punch (Suffolk) (horse) PUN + CH(inese)