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' says to put letters next to each other.
'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" .)