Old man disturbed in tree (6)
I believe the answer is:
almond
'tree' is the definition.
(almond tree is a kind of tree)
'old man disturbed' is the wordplay.
'disturbed' indicates anagramming the letters.
'old'+'man'='oldman'
'oldman' is an anagram of 'ALMOND'.
'in' acts as a link.
(Other definitions for almond that I've seen before include "Kind of nut, makes nice icing on cake" , "Yellowish brown" , "Nut used in marzipan" , "Nut (tree)" , "Nut which provides cake icing" .)