Mother left permit for hammer (6)
I believe the answer is:
mallet
'for hammer' is the definition.
(mallet is a kind of hammer)
'mother left permit' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'left' becomes 'l' (common abbreviation).
'permit' becomes 'let' (synonyms).
'ma'+'l'+'let'='MALLET'
(Other definitions for mallet that I've seen before include "Wooden-headed hammer" , "Used to beat" , "Type of hammer" , "It's used in croquet and polo" , "Croquet stick" .)