Silly to eat small nuts (6)
I believe the answer is:
insane
'nuts' is the definition.
(similar in meaning)
'silly to eat small' is the wordplay.
'silly' becomes 'inane' (I've seen this before).
'to eat' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'inane' going around 's' is 'INSANE'.
(Other definitions for insane that I've seen before include "Round the bend" , "Stupid" , "Mental" , "having a screw loose" , "reckless" .)