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