Questionable American Pastor infiltrating religious group (7)
I believe the answer is:
suspect
'questionable' is the definition.
(similar in meaning)
'american pastor infiltrating religious group' is the wordplay.
'american' becomes 'US' (United States).
'pastor' becomes 'p' (abbreviation).
'infiltrating' means one lot of letters goes inside another.
'religious group' becomes 'sect' (eg a religious sect).
'us'+'p'='usp'
'usp' going inside 'sect' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Imagine to be guilty" , "Presume" , "One thought to have committed crime" , "Dubious" , "Be ready to believe - one of the usual crowd?" .)