Betray mum and ring head of police (4)
I believe the answer is:
shop
'betray' is the definition.
(thesaurus)
'mum and ring head of police' is the wordplay.
'mum' becomes 'sh' (I've seen this in other clues).
'and' says to put letters next to each other.
'ring' becomes 'o'.
'head of' suggests taking the first letters.
The first letter of 'police' is 'p'.
'sh'+'o'+'p'='SHOP'
(Other definitions for shop that I've seen before include "Posh (anag.)" , "Retail establishment" , "Place to buy" , "Inform on (slang)" , "Inform on; store" .)