In the middle of ballroom, I wait in line, briefly, for drink (7)
I believe the answer is:
liqueur
'for drink' is the definition.
(liqueur is a kind of drink)
'in the middle of ballroom i wait in line briefly' is the wordplay.
'in the' means one lot of letters goes inside another.
'middle of' says to take the centre.
'wait in line' becomes 'queue' (I've seen this before).
'briefly' means to remove the last letter.
The middle of 'ballroom' is 'lr'.
'i'+'queue'='iqueue'
'iqueue' with its last letter taken away is 'iqueu'.
'lr' enclosing 'iqueu' is 'LIQUEUR'.
(Other definitions for liqueur that I've seen before include "Alcoholic chocolate?" , "Sweet flavoured spirit" , "After-dinner drink" , "E.g. cherry brandy" , "Chocolate filled with alcohol" .)