A scam involving first of ripe fruit (5)
I believe the answer is:
acorn
'fruit' is the definition.
(I know that acorn is a type of nut)
'a scam involving first of ripe' is the wordplay.
'scam' becomes 'con' (both can mean a fraudulent scheme).
'involving' is an insertion indicator.
'first of ripe' becomes 'r' (1st letter of 'ripe').
'con' enclosing 'r' is 'corn'.
'a'+'corn'='ACORN'
(Other definitions for acorn that I've seen before include "Model in stone" , "From which a tall tree may grow" , "Fruit of the oak tree" , "Nut of oak tree" , "From which a mighty oak may grow?" .)