A number going to church for fish (5)
I believe the answer is:
tench
'for fish' is the definition.
(tench is a kind of fish)
'a number going to church' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'going to' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
'ten'+'ch'='TENCH'
(Other definitions for tench that I've seen before include "something fishy" , "Coarse fish" , "Swimmer" , "One in school" , "catch" .)