Bag up very cautious bird (9)
I believe the answer is:
cassowary
'bird' is the definition.
(cassowary is a kind of bird)
'bag up very cautious' is the wordplay.
'bag' becomes 'sac' (a small bag).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'very' becomes 'so' (both can mean 'extremely').
'cautious' becomes 'wary' (similar in meaning).
'sac' in reverse letter order is 'cas'.
'cas'+'so'+'wary'='CASSOWARY'
(Other definitions for cassowary that I've seen before include "Winger" , "So a cry was changed of the running bird" , "Flightless bird related to the emu - Oscar's way (anag)" , "Large black flightless bird of N Australia" , "Large flightless bird - Oscar's way (anag)" .)