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' means one lot of letters goes inside another.
'party' becomes 'sect' (I've seen this in another clue).
'us'+'p'='usp'
'usp' placed within '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" .)