Old, burning tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I've seen this before)
'old burning' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'burning' becomes 'live' ('live' can be similar in meaning to 'burning').
'o'+'live'='OLIVE'
(Other definitions for olive that I've seen before include "woman" , "Oil-yielding tree" , "Yellowish-green colour" , "It's branch is a symbol of peace" , "Oily stone fruit" .)