Fun bird (4)
I believe the answer is:
lark
This clue is a double definition.
'fun' is the first definition.
(what a lark)
'bird' is the second definition.
(lark is a kind of bird)
(Other definitions for lark that I've seen before include "what one may get up to?" , "carefree adventure" , "Frolic - songbird" , "High-flying songbird" , "Foolish activity, songbird" .)