Old man turned out to be nut (6)
I believe the answer is:
almond
'to be nut' is the definition.
(almond is a type of nut)
'old man turned' is the wordplay.
'turned' is an anagram indicator.
'old'+'man'='oldman'
'oldman' is an anagram of 'ALMOND'.
'out' is the link.
(Other definitions for almond that I've seen before include "Nut often used in cake icing (6)." , "Fruit kernel of a peach-related tree" , "Old man with nice nut" , "Oval-shaped nut" , "Marzipan-producing nut" .)