Dodgy American quietly infiltrates party (7)
I believe the answer is:
suspect
'dodgy' is the definition.
(I've seen this before)
'american quietly infiltrates party' is the wordplay.
'american' becomes 'US' (United States).
'quietly' becomes 'p' (musical symbol).
'infiltrates' is an insertion indicator.
'party' becomes 'sect' (I've seen this before).
'us'+'p'='usp'
'usp' inserted into 'sect' is 'SUSPECT'.
(Other definitions for suspect that I've seen before include "Possibly dangerous or false" , "Be ready to believe - one of the usual crowd?" , "Have misgivings about" , "Believe (something) possible or probable" , "Imagine guilt" .)