Bird caught while very alert (9)
I believe the answer is:
cassowary
'bird' is the definition.
(cassowary is a kind of bird)
'caught while very alert' is the wordplay.
'caught while' becomes 'cas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'very' becomes 'so' (both can mean 'extremely').
'alert' becomes 'wary' (similar in meaning).
'cas'+'so'+'wary'='CASSOWARY'
Can you help me to learn more?
(Other definitions for cassowary that I've seen before include "So a cry was changed of the running bird" , "Large flightless bird - Oscar's way (anag)" , "Large black flightless bird of N Australia" , "Winger" , "Flightless bird related to the emu - Oscar's way (anag)" .)