Simpleton: silly old one (6)
I believe the answer is:
noodle
'simpleton' is the definition.
(I've seen this before)
'silly old one' is the wordplay.
'silly' indicates anagramming the letters.
'old'+'one'='oldone'
'oldone' with letters rearranged gives 'NOODLE'.
(Other definitions for noodle that I've seen before include "Idiot" , "Pasta strip; silly person" , "Nitwit" , "Simpleton - strip of pasta" , "Simpleton; piece of pasta" .)