Friend mum's cut short (4)
I believe the answer is:
mate
'friend' is the definition.
(mate is a kind of friend)
'mum's cut short' is the wordplay.
'mum' becomes 'mater' (old-fashioned term for mother).
'cut short' means to remove the last letter.
'mater' with its final letter taken away is 'MATE'.
(Other definitions for mate that I've seen before include "Spouse or informal pal (4)" , "Pal, chum" , "Match" , "Fellow member, partner" , "First officer" .)