Scoundrels swear endlessly (4)
I believe the answer is:
curs
'scoundrels' is the definition.
(I've seen this in another clue)
'swear endlessly' is the wordplay.
'swear' becomes 'curse' (synonyms).
'endlessly' means to remove the last letter.
'curse' with its final letter taken off is 'CURS'.
(Other definitions for curs that I've seen before include "Mongrel dogs" , "Scoundrels; dogs" .)