Small wine for fun (5)
I believe the answer is:
sport
'fun' is the definition.
('sport' can be a synonym of 'fun')
'small wine' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'wine' becomes 'port' (port is a kind of wine).
's'+'port'='SPORT'
'for' is the link.
(Other definitions for sport that I've seen before include "form of exercise?" , "Wear - game - generous type" , "Show off" , "Eg, rugby" , "Abnormal" .)