Dubious American priest admitted to cult (7)
I believe the answer is:
'dubious' is the definition.
(I've seen this before)
'american priest admitted to cult' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'priest' becomes 'p' (ecclesiastical abbreviation).
'admitted to' indicates putting letters inside.
'cult' becomes 'sect' (religious sect).
'usp' going into 'sect' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Hazard a guess" , "Doubt - man held by police" , "Fishy" , "man distrusted?" , "Possible perpetrator of a crime" .)