Be underneath old tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I know that olive is a type of tree)
'be underneath old' is the wordplay.
'be' becomes 'live' (to be is to live or exist).
'underneath' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'old' becomes 'o' (abbreviation).
'live' put after 'o' is 'OLIVE'.
(Other definitions for olive that I've seen before include "One's stoned" , "Green shade; its branch for peace" , "woman" , "Oil producer" , "Greyish-green colour" .)