Untrustworthy American president primarily in the grip of extreme body of followers (7)
I believe the answer is:
suspect
'untrustworthy' is the definition.
'american president primarily in the grip of extreme body of followers' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'primarily' suggests taking the first letters.
'in the grip of' indicates putting letters inside.
'extreme body of followers' becomes 'sect' (as in a cult).
The first letter of 'president' is 'p'.
'us'+'p'='usp'
'usp' inserted within 'sect' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Fancy" , "Believe (someone) guilty" , "Consider or believe likely" , "One believed to have committed a crime" , "Imagine to be guilty" .)