Tree old as it happens (5)
I believe the answer is:
olive
'tree' is the definition.
(olive is a kind of tree)
'old as it happens' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'as it happens' becomes 'live' (as in live broadcasting).
'o'+'live'='OLIVE'
(Other definitions for olive that I've seen before include "Yellowish-green colour" , "girl" , "Oil-yielding fruit" , "Tree - shade of green" , "A Med. fruit-tree" .)