Enthusiast who gets nothing right (5)
I believe the answer is:
lover
'enthusiast' is the definition.
(someone who loves something)
'nothing right' is the wordplay.
'nothing' becomes 'love' (love means zero in tennis).
'right' becomes 'r' (abbreviation).
'love'+'r'='LOVER'
'who gets' acts as a link.
(Other definitions for lover that I've seen before include "I could be fickle" , "finished litre first" , "Opposite of hateful type" , "Sweetheart" , "Devotee" .)