Questionable cult defends American power (7)
I believe the answer is:
suspect
'questionable' is the definition.
('suspect' can be similar in meaning to 'questionable')
'cult defends american power' is the wordplay.
'cult' becomes 'sect' (I've seen this before).
'defends' means one lot of letters goes inside another.
'american' becomes 'us'.
'power' becomes 'p'.
'us'+'p'='usp'
'sect' going around 'usp' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Funny" , "Have reason to believe" , "Imagine to be true" , "Presume" , "Person being accused" .)