Hardy partner is a bay tree! (6)
I believe the answer is:
laurel
I believe this clue is a double definition.
'hardy partner' is the first definition.
(I've seen this in another clue)
'a bay tree' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for laurel that I've seen before include "girl" , "Evergreen tree also called bay" , "Hardy's partner - symbol of victory" , "Does A1 rule this bush?" , "Companion to Hardy" .)