Raised threat, so ornithologist hides nest (5)
I believe the answer is:
roost
'nest' is the definition.
(I've seen this in another clue)
'raised threat so ornithologist hides' is the wordplay.
'raised' says the letters should be written backwards (going up in a down clue).
'hides' indicates a hidden word.
'tsoor' is hidden within 'threat so ornithologist'.
'tsoor' back-to-front is 'ROOST'.
(Other definitions for roost that I've seen before include "Settle down (on a branch?)" , "where fliers land" , "It's sometimes ruled" , "Place where birds rest" , "(Bird) settle for sleep" .)