Mum rings off! (4)
I believe the answer is:
shoo
'mum rings off' is the definition.
I can't judge whether this definition defines the answer.
'mum rings' is the wordplay.
'mum' becomes 'sh' (both can mean to be quiet).
'rings' becomes 'oo' (two O's - O resembles a ring).
'sh'+'oo'='SHOO'
Can you help me to learn more?
(Other definitions for shoo that I've seen before include "Push off" , "Dismissive remark" , "Go away quickly!" , "used to scare away animals" , "Begone!" .)