Tree needing oxygen as it happens (5)
I believe the answer is:
olive
'tree' is the definition.
(I've seen this before)
'oxygen as it happens' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'as it happens' becomes 'live' (as in live television).
'o'+'live'='OLIVE'
'needing' is the link.
(Other definitions for olive that I've seen before include "Small green Mediterranean tree fruit, nice in martini" , "Green shade; its branch for peace" , "Yellowish-green - fruit" , "Fruit giving oil" , "Cocktail fruit" .)