Something witty from mum? Not her! (3)
I believe the answer is:
mot
'something witty' is the definition.
(as in a 'bon mot')
'mum? not her' is the wordplay.
'mum?' becomes 'mother' (mum is a kind of mother).
'not' is a deletion indicator.
'mother' with 'her' taken away is 'MOT'.
'from' is the link.
(Other definitions for mot that I've seen before include "A French word, may be bon" , "Witty words" , "Vehicle check" , "witty remark" , "type of test" .)