Fish very important in church (7)
I believe the answer is:
sockeye
'fish' is the definition.
(sockeye is a kind of fish)
'very important in church' is the wordplay.
'very' becomes 'so' (I've seen this in other clues).
'important' becomes 'key' ('key' can be similar in meaning to 'important').
'in' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'key' going within 'ce' is 'ckeye'.
'so'+'ckeye'='SOCKEYE'
(Other definitions for sockeye that I've seen before include "Type of Pacific salmon" , "Breed of salmon" , "Swimmer" , "Commercially important salmon" , "Small northern Pacific salmon" .)