Volunteers drank concoction in mug (7)
I believe the answer is:
tankard
'mug' is the definition.
(I've seen this in another clue)
'volunteers drank concoction in' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'concoction' indicates an anagram.
'in' means one lot of letters goes inside another.
'drank' with letters rearranged gives 'nkard'.
'ta' placed inside 'nkard' is 'TANKARD'.
(Other definitions for tankard that I've seen before include "Drinking vessel with a handle" , "Drank at (anag)" , "Tall beer mug" , "Beer glass" , "One-handled beer mug" .)