Mum's eaten nuts for fun (9)
I believe the answer is:
amusement
'fun' is the definition.
(I've seen this before)
'mum's eaten nuts' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'mums'+'eaten'='mumseaten'
'mumseaten' anagrammed gives 'AMUSEMENT'.
'for' acts as a link.
(Other definitions for amusement that I've seen before include "Fun" , "(State of) finding something funny" , "enjoyment for 19 > [(type of) PARK]" , "Diversion, recreation" , "Pastime; recreation" .)