Foul-mouthed American quartet in Lincoln? (7)
I believe the answer is:
abusive
'foul-mouthed' is the definition.
(I've seen this before)
'american quartet in lincoln?' is the wordplay.
'american' becomes 'us'.
'quartet' becomes 'iv' (roman numerals for four).
'in' is an insertion indicator.
'lincoln?' becomes 'abe'.
'us'+'iv'='usiv'
'usiv' inserted into 'abe' is 'ABUSIVE'.
(Other definitions for abusive that I've seen before include "Reviling with coarse language" , "scathing" , "Insulting, being harshly critical or offensive" , "Calumniating" , "Swearing, insulting" .)