Clearly, old man is a nut (6)
I believe the answer is:
almond
'nut' is the definition.
(I know that almond is a type of edible nut)
'clearly old man' is the wordplay.
'old man' anagrams to 'ALMOND'.
However, I am not clear how this is indicated.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for almond that I've seen before include "Fruit kernel of a peach-related tree" , "Nutlike seed" , "Nut often used in cake icing (6)." , "Kind of nut, makes nice icing on cake" , "Nut (tree)" .)