Volunteers drank, used small mugs (8)
I believe the answer is:
tankards
'mugs' is the definition.
(I've seen this in another clue)
'volunteers drank used small' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'used' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'drank' with letters rearranged gives 'nkard'.
'ta'+'nkard'+'s'='TANKARDS'
(Other definitions for tankards that I've seen before include "Large one-handled drinking vessels" , "Large one-handled drinking mugs" , "One-handled beer mugs" , "Large beer mugs" .)