Dwell under old tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I know that Olive is a type of olive tree)
'dwell under old' is the wordplay.
'dwell' becomes 'live' (synonyms).
'under' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'live' after 'o' is 'OLIVE'.
(Other definitions for olive that I've seen before include "Yellowish-green colour" , "Bitter fruit" , "Girl's name; tree" , "A Med. fruit-tree" , "Mediterranean tree-fruit - nice in martini" .)