Old man's crackers or nuts (7)
I believe the answer is:
almonds
'nuts' is the definition.
(almond is a kind of nut)
'old man's crackers' is the wordplay.
'crackers' indicates an anagram (informal term for mad).
'old'+'mans'='oldmans'
'oldmans' anagrammed gives 'ALMONDS'.
'or' is the link.
(Other definitions for almonds that I've seen before include "Fruits for marzipan" , "Sold man on kind of nuts" , "Man sold (anag.)" , "Nuts used to make marzipan" .)