Mum's hard potatoes (4)
I believe the answer is:
mash
'potatoes' is the definition.
(I've seen this before)
'mum's hard' is the wordplay.
'mum's' becomes 'mas' ('ma' can be a synonym of 'mum').
'hard' becomes 'h'.
'mas'+'h'='MASH'
(Other definitions for mash that I've seen before include "Black comedy film set in Korean War" , "dish of veg" , "food for animals" , "Crush, potatoes say" , "Animal feed" .)