Swear after mate creates a fuss (7)
I believe the answer is:
palaver
'a fuss' is the definition.
(I've seen this before)
'swear after mate' is the wordplay.
'swear' becomes 'aver' ('aver' can be a synonym of 'swear').
'after' means one lot of letters go next to another.
'mate' becomes 'pal' (I've seen this before).
'aver' put after 'pal' is 'PALAVER'.
'creates' is the link.
(Other definitions for palaver that I've seen before include "Song-and-dance" , "Rigmarole" , "Powwow - a lot of fuss" , "Long tedious fuss" , "Commotion" .)