Tree: source of oranges, as it happens (5)
I believe the answer is:
olive
'tree' is the definition.
(I've seen this before)
'source of oranges as it happens' is the wordplay.
'source of' indicates taking the first letters.
'as it happens' becomes 'live' (as in live television).
The first letter of 'oranges' is 'o'.
'o'+'live'='OLIVE'
(Other definitions for olive that I've seen before include "Fruit also used to make oil" , "Mediterranean tree-fruit, nice in cocktail" , "This branch is offered to show peace" , "Source of edible oil" , "Mediterranean fruit or greenish colour" .)