Bird first-rate seen climbing locust tree, perhaps (7)
I believe the answer is:
robinia
'locust tree perhaps' is the definition.
(locust trees belong to the genus Robinia)
'bird first-rate seen climbing' is the wordplay.
'bird' becomes 'robin' (robin is a kind of bird).
'first rate' becomes 'AI' (resembles A1, old-fashioned slang for very good).
'seen climbing' says the letters should be written backwards (in a down clue, letters climb up).
'ai' reversed gives 'ia'.
'robin'+'ia'='ROBINIA'
(Other definitions for robinia that I've seen before include "Plant" , "Shrub of the false acacia genus" .)