May tree (3)
'may tree' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hawthorn that I've seen before include "plant" , "Common hedge bush" , "May-flowering small tree" , "Hedge-tree" , "fruit supplier" .)
(Other definitions for haw that I've seen before include "Hedgerow fruit" , "A blackthorn berry" , "Fruit of common thorn tree (3)" , "Berry of a thorny shrub" , "Fruit of the hawthorn" .)