Straight punch catching back of Fulham supporter (4)
I believe the answer is:
jamb
'supporter' is the definition.
(post supporting a door)
'straight punch catching back of fulham' is the wordplay.
'straight punch' becomes 'jab' (jab can mean to punch or poke).
'catching' means one lot of letters goes inside another.
'back of' says to take the final letters.
The final letter of 'fulham' is 'm'.
'jab' enclosing 'm' is 'JAMB'.
(Other definitions for jamb that I've seen before include "Vertical side of a doorway" , "bit on the side?" , "Doorpost" , "Door, window side post" , "Upright sidepiece" .)