Fifty-one line up endlessly on right for a drink (7)
I believe the answer is:
liqueur
'a drink' is the definition.
(liqueur is a kind of drink)
'fifty-one line up endlessly on right for' is the wordplay.
'fifty one' becomes 'li' (Roman numeral).
'line up' becomes 'queue' ('queue' can be a synonym of 'line up').
'endlessly' means to remove the last letter.
'on' says to put letters next to each other.
'right for' becomes 'r' (I've seen this in other clues).
'queue' with its last letter taken away is 'queu'.
'li'+'queu'+'r'='LIQUEUR'
(Other definitions for liqueur that I've seen before include "E.g. cherry brandy" , "After-dinner drink" , "Curacao, say" , "Postprandial drink" , "eg Benedictine" .)