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' (abbreviation).
'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" .)