Nut loaf remnant (8)
I believe the answer is:
headbutt
'nut' is the definition.
(nut can informally mean to headbutt)
'loaf remnant' is the wordplay.
'loaf' becomes 'head' (I've seen this in other clues).
'remnant' becomes 'butt' (the remnant of a cigarette).
'head'+'butt'='HEADBUTT'
(Other definitions for headbutt that I've seen before include "nut" , "personal assault" .)