The head's a funny old one! (6)
I believe the answer is:
noodle
'the head's' is the definition.
(noodle is a kind of head)
'a funny old one' is the wordplay.
'a funny' indicates anagramming the letters.
'old'+'one'='oldone'
'oldone' with letters rearranged gives 'NOODLE'.
(Other definitions for noodle that I've seen before include "Goose" , "Narrow strip of pasta, or foolish person" , "Slangy head or piece of pasta" , "Slangy head or pasta strip" , "Dope - put in the soup" .)