Church activity declining, mostly trendy (7)
I believe the answer is:
worship
'church activity' is the definition.
(I've seen this in another clue)
'declining mostly trendy' is the wordplay.
'declining' becomes 'worse' (declining means getting worse).
'mostly' means to remove the last letter (most but not all of the word).
'trendy' becomes 'hip' (both can mean fashionable).
'worse' with its final letter taken away is 'wors'.
'wors'+'hip'='WORSHIP'
(Other definitions for worship that I've seen before include "Love" , "prayer" , "Religious devotion" , "Adore, adulate" , "Revere" .)