I found this on the easy side – solving time, 11 mins.
* = anagram < = reversed
ACROSS
1 WOLF RAM
5 C (HIS) EL(l) his = the man’s prison room = cell
10 SETT(L)ER L = 50 in Roman numerals
11 A P (PO) INT A two-letter river, well-known to solvers
12 LIS(Z)T Variable (from algebra) can be x, y, or (as here) z
13 I’M PARTING
14 OBSOLESCENT (Celeb’s not so)*
18 TRA(i)NS FORMER
21 APPETISER (Peter’s pie)*
23 NAIRA Hidden reversal – basic currency unit of Nigeria
24 INRO ADS
25 IRON OUT Golf club = iron. In normal golf round of 18 holes, the first 9 are known as ‘out’
26 REA SEA cf seared
27 DATA SET A (article) in (stated)*
DOWN
1 WI(SE)LY
2 LU (TI) ST it<
3 R ELATION S parson’s heart = RS (middle letters)
4 MARTIN SCORSESE (minor actresses)*
6 HYPER Cleverly hidden
7 SPIRITED (priest I’d)*
8 LIT I GATE lit = vandalised, I think. Not sure if they are exactly equivalent
9 HAPPILY MARRIED Not a dict phrase, I think, but very familiar. Cryptic definition
15 SP (RING) OUT
16 S (second) TEAM (side) I (one) ER (first letters)
17 H (A MP) E RED
19 MINORS “Miners”
20 PA’S T IT
22 T (first letter of threaten) RACE