Tree ointment when scratching bottom (5)
I believe the answer is:
balsa
'tree' is the definition.
(balsa is a kind of tree)
'ointment when scratching bottom' is the wordplay.
'ointment' becomes 'balsam' (balsam is a kind of ointment).
'when scratching bottom' means to remove the last letter (remove bottom letter in down clue).
'balsam' with its final letter taken off is 'BALSA'.
(Other definitions for balsa that I've seen before include "Wood used in model making" , "Something light" , "Tropical American tree" , "wood for raft" , "South American tree" .)