Drink concession given up by firm (6)
I believe the answer is:
posset
'drink' is the definition.
(I know that posset is a type of drink)
'concession given up by firm' is the wordplay.
'concession' becomes 'sop' (sop is a kind of concession).
'given up' says the letters should be written in reverse (in a down clue, letters go up).
'by' is a charade indicator (letters next to each other).
'firm' becomes 'set' (both can mean fixed).
'sop' in reverse letter order is 'pos'.
'pos'+'set'='POSSET'
(Other definitions for posset that I've seen before include "So step on curdled milk once used as a cold remedy" , "Curdled milk drink" , "Sweet spiced milk curdled with alcohol" , "As relief for cold" .)