Old man cracking a nut (6)
I believe the answer is:
almond
'a nut' is the definition.
(almond is a type of nut)
'old man cracking' is the wordplay.
'cracking' is an anagram indicator.
'old'+'man'='oldman'
'oldman' anagrammed gives 'ALMOND'.
(Other definitions for almond that I've seen before include "Oval-shaped nut" , "Edible nut" , "Nut often used in cake icing (6)." , "Nut-like kernel" , "Marzipan-producing nut" .)