A no-good American chap? (5)
I believe the answer is:
angus
'chap?' is the definition.
'a no-good american' is the wordplay.
'no good' becomes 'ng' (abbreviation for no and good).
'american' becomes 'us' (abbreviation for United States).
'a'+'ng'+'us'='ANGUS'
(Other definitions for angus that I've seen before include "Scotsman" , "Scottish cattle area" , "Scot" , "Aberdeen . . . . ., breed of cattle" , "Lamb of God - . . . Dei" .)