Go behind congregation, mostly depressed (6)
I believe the answer is:
follow
'go behind' is the definition.
(I've seen this before)
'congregation mostly depressed' is the wordplay.
'congregation' becomes 'fold' (as in welcoming someone into 'the fold').
'mostly' means to remove the last letter (most but not all of the word).
'depressed' becomes 'low' (synonyms).
'fold' with its final letter taken away is 'fol'.
'fol'+'low'='FOLLOW'
(Other definitions for follow that I've seen before include "Come next" , "Go along (a route)" , "Go behind" , "Chase" , "Succeed; go after" .)