Old man upset by something eaten (6)
I believe the answer is:
almond
'something eaten' is the definition.
I can't tell whether this definition defines the answer.
'old man upset' is the wordplay.
'upset' indicates an anagram.
'old'+'man'='oldman'
'oldman' anagrammed gives 'ALMOND'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for almond that I've seen before include "Nut which provides cake icing" , "Nut (tree)" , "one of the nuts" , "Fruit for marzipan" , "Type of nut" .)