Not too difficult. Solving time, 16 mins.
* = anagram < = reversed
ACROSS
1 U MP IRE
4 OB SERVER Nice seamless join at see/old
9 CHE RUB
10 TORT ILL A
11 ON E-SHOT (ethos)*
13 G (R) UY ERE
14 HAVING KITTENS Cryptic definition – very good
17 DISHONOURABLE (unholier bod as)*
21 CAST OFF cast = actors
23 NO (URI) SH
24 RINGTONE resonating = (a ringtone’s)*
25 E D WARD
26 DI SCREET (secret)*
27 S P L EEN
DOWN
1 UN CLOT HE
2 P RESERVED
3 ROU G HEN (our)*
5 BROUGHT DOWN
6 ENTHUSE Hidden
7 VAL (U) E
8 REA (DE) R ed<
12 TAKE OF FENCE
15 SUBLIMATE (but meal is)*
16 MEN H ADEN
18 S COOT ER
19 ROUND-UP Round up (to get more)
20 A CAR(I)D
22 SON GS Gilbert and Sullivan