I found this fairly easy, with the two 15-letters entries solved straightaway. Solving time, 15 mins.
* = anagram
ACROSS
1 LES SON
4 MAR ROW
9 STABLEMATE Cryptic definition
10 CAM P
11 BENEFICE (nice beef)*
12 IN VENT
13 DE (LIGHT) ED
15 REIN “Rain”
16 C(Y) AN
17 TAKE SIDES
20 LABOUR Double definition
21 PL ATONIC (action)*
23 GNUS cf guns
24 OLEAGINO US (I go alone)*
25 Sir Richard STEELE “steal”
26 BRID(L)E L = 50 in Roman numerals
DOWN
1 LATTER-DAY SAINTS (try deal satanist)*
2 SABRE hidden reversal (going up)
3 OP EN(d)ING
5 A M (ER) IND
6 RECOVERED Double definition
7 WOMENS INSTITUTE (won’t meet in suits)*
8 LAME NT
14 LA NGOUSTE (one’s gut)*
17 T (URM) OIL (rum)*
18 KI (PP) ER
19 STAG GE(a)R
22 OWNED Double definition