Birds having fun? (5)
I believe the answer is:
larks
'birds' is the definition.
(lark is a kind of bird)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for larks that I've seen before include "[EARLY BIRD]?" , "Japes" , "Small songbirds" , "Fun and games" , "Flyers who tweet" .)