Grimace when church pursues gain (5)
I believe the answer is:
wince
'grimace' is the definition.
(wincing is a kind of grimacing)
'church pursues gain' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'pursues' is a charade indicator (letters next to each other).
'gain' becomes 'win' (synonyms).
'ce' after 'win' is 'WINCE'.
'when' is the link.
(Other definitions for wince that I've seen before include "Visibly react to pain" , "Pull a face" , "Flinch with pain" , "Make face showing pain" , "Start painfully" .)