Bird caught on line (4)
I believe the answer is:
crow
'bird' is the definition.
(crow is a kind of bird)
'caught on line' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'line' becomes 'row' (a row or line of things).
'c'+'row'='CROW'
(Other definitions for crow that I've seen before include "shout in triumph" , "Brave" , "Boast, swagger, triumph" , "bar" , "One joins others in a murder" .)