Make tough old queen a Liberal (6)
I believe the answer is:
anneal
'make tough' is the definition.
(annealing is a kind of making tough)
'old queen a liberal' is the wordplay.
'old queen' becomes 'anne' (Queen Anne).
'a liberal' becomes 'al' (I've seen this in other clues).
'anne'+'al'='ANNEAL'
(Other definitions for anneal that I've seen before include "Destress using heat" , "Heat and cool slowly in order to toughen" , "Temper by heat treatment" , "Toughen, temper (metal)" , "Heat-treat (metal or glass)" .)