Long-legged creature burning oxygen (8)
I believe the answer is:
flamingo
'long-legged creature' is the definition.
'burning oxygen' is the wordplay.
'burning' becomes 'flaming' (flaming is a kind of burning).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'flaming'+'o'='FLAMINGO'
(Other definitions for flamingo that I've seen before include "Long-legged wader" , "Pretty bird" , "Exotic bird" , "Large pink bird" , "Long-legged bird" .)