Mum's eaten bananas for fun (9)
I believe the answer is:
amusement
'for fun' is the definition.
(amusement is fun or enjoyment)
'mum's eaten bananas' is the wordplay.
'bananas' is an anagram indicator (bananas can informally mean crazy).
'mums'+'eaten'='mumseaten'
'mumseaten' with letters rearranged gives 'AMUSEMENT'.
(Other definitions for amusement that I've seen before include "Entertainment" , "Light-hearted pastime" , "Pastime; recreation" , "Diversion, recreation" , "Something which entertains" .)