Mum shortly becomes a friend (4)
I believe the answer is:
mate
'a friend' is the definition.
(mate is a kind of friend)
'mum shortly' is the wordplay.
'mum' becomes 'mater' (old-fashioned term for mother).
'shortly' means to remove the last letter.
'mater' with its final letter taken off is 'MATE'.
'becomes' acts as a link.
(Other definitions for mate that I've seen before include "One of a pair or a pal" , "Marriage partner" , "Friend, companion" , "Final check at chess" , "Ship's officer or marital partner" .)