Enthusiast left, finished (5)
I believe the answer is:
lover
'enthusiast' is the definition.
(someone who loves something)
'left finished' is the wordplay.
'left' becomes 'l' (common abbreviation).
'finished' becomes 'over' (similar in meaning).
'l'+'over'='LOVER'
(Other definitions for lover that I've seen before include "toy boy" , "Swain" , "finished litre first" , "Sweetheart" , "Romantic partner" .)