Old woman's hot brew (4)
I believe the answer is:
mash
'brew' is the definition.
(I've seen this before)
'old woman's hot' is the wordplay.
'old woman's' becomes 'mas' (belongs to ma).
'hot' becomes 'h' (abbreviation eg on taps).
'mas'+'h'='MASH'
(Other definitions for mash that I've seen before include "Crush, potatoes say" , "dish of veg" , "Forces hospital" , "Brew or infuse (tea)" , "Sham sort of pulp" .)