One old silly blockhead (6)
I believe the answer is:
noodle
'blockhead' is the definition.
(I've seen this before)
'one old silly' is the wordplay.
'silly' is an anagram indicator.
'one'+'old'='oneold'
'oneold' is an anagram of 'NOODLE'.
(Other definitions for noodle that I've seen before include "Egg pasta" , "See silly person" , "Blockhead" , "Form of pasta" , "Pasta strip; silly person" .)