Fruit upset old man (6)
I believe the answer is:
almond
'fruit' is the definition.
(almond is a kind of fruit)
'upset old man' is the wordplay.
'upset' is an anagram indicator.
'old'+'man'='oldman'
'oldman' with letters rearranged gives 'ALMOND'.
(Other definitions for almond that I've seen before include "Nut used in marzipan" , "Kind of nut, makes nice icing on cake" , "Nut which provides cake icing" , "Old man with nice nut" , "Nut often used in cake icing (6)." .)