A great time is had with leftover rum (7)
I believe the answer is:
oddball
'rum' is the definition.
(rum can mean strange)
'a great time is had with leftover' is the wordplay.
'a great time' becomes 'ball' ('having a ball' can mean having a great time).
'is had with' is a charade indicator (letters next to each other).
'leftover' becomes 'odd' ('odd' can be a synonym of 'leftover').
'ball' put after 'odd' is 'ODDBALL'.
(Other definitions for oddball that I've seen before include "Eccentric person (colloq.)" , "Eccentric person like peculiar sphere" , "eccentric type" , "Strange person (colloq.)" , "Nonconformist - peculiar" .)