Hero's lover beneath old tree (8)
I believe the answer is:
oleander
'tree' is the definition.
(I've seen this before)
'hero's lover beneath old' is the wordplay.
'hero's lover' becomes 'leander' (as in Hero and Leander in Greek myth).
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'leander' after 'o' is 'OLEANDER'.
(Other definitions for oleander that I've seen before include "Red alone (anag.)" , "No leader, oddly, for the flowering shrub" , "Poisonous evergreen shrub with white, pink or red flowers" , "Evergreen flowering shrub from olden era" , "Adore Len (anag.)" .)