Tame animal climbing tree may end scare (7)
I believe the answer is:
petrify
'scare' is the definition.
(thesaurus)
'tame animal climbing tree may end' is the wordplay.
'tame animal' becomes 'pet' (pets are tamed animals).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'tree' becomes 'fir' (fir is a kind of tree).
'end' indicates one should take the final letters.
The last letter of 'may' is 'y'.
'fir' written backwards gives 'rif'.
'pet'+'rif'+'y'='PETRIFY'
(Other definitions for petrify that I've seen before include "Scare stiff" , "Turn to stone" , "Paralyse with fear" , "Frighten stiff" .)