Splinter group admitting American power is questionable (7)
I believe the answer is:
suspect
'questionable' is the definition.
(similar in meaning)
'splinter group admitting american power' is the wordplay.
'splinter group' becomes 'sect' (I've seen this before).
'admitting' means one lot of letters goes inside another.
'american' becomes 'us' (abbreviation for United States).
'power' becomes 'p'.
'us'+'p'='usp'
'sect' going around 'usp' is 'SUSPECT'.
'is' acts as a link.
(Other definitions for suspect that I've seen before include "Believe to be guilty, but without proof" , "Believe (something) possible or probable" , "Be ready to believe - one of the usual crowd?" , "One thought to have committed crime" , "Doubt truth of" .)