Impact of popular complaint in French church (9)
I believe the answer is:
influence
'impact' is the definition.
(influencing is a kind of impacting)
'popular complaint in french church' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'complaint' becomes 'flu' (flu is a kind of health problem).
'in french' becomes 'en' ('in' in French).
'church' becomes 'CE' (abbreviation for Church of England).
'in'+'flu'+'en'+'ce'='INFLUENCE'
'of' is the link.
(Other definitions for influence that I've seen before include "Weight" , "The capacity to affect others" , "The prestige and power to affect others" , "Effect" , "Clue in fen (anag.)" .)