Old man turned out to be a nut (6)
I believe the answer is:
almond
'a nut' is the definition.
(almond is a type of nut)
'old man turned out' is the wordplay.
'turned out' indicates anagramming the letters (turn out can mean to organise).
'old'+'man'='oldman'
'oldman' is an anagram of 'ALMOND'.
'to be' is the link.
(Other definitions for almond that I've seen before include "Nutlike seed" , "Nut which provides cake icing" , "one of the nuts" , "Type of nut" , "Yellowish brown" .)