Drink to small hairpiece (4)
I believe the answer is:
swig
'drink to' is the definition.
(thesaurus)
'small hairpiece' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hairpiece' becomes 'wig' (wig is a kind of hairpiece).
's'+'wig'='SWIG'
(Other definitions for swig that I've seen before include "Large swallow" , "Swallow (drink) greedily" , "(Take) quick drink from bottle" , "Large draught of drink" , "Drink (a deep draught)" .)