Swamp creature caught enormous bird (4)
I believe the answer is:
croc
'swamp creature' is the definition.
(short for crocodile)
'caught enormous bird' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'enormous bird' becomes 'roc' (mythical giant bird).
'c'+'roc'='CROC'
(Other definitions for croc that I've seen before include "Fierce beast" , "Large predatory reptile (colloq.)" , "African beast; sounds like piece of pot" , "Pot tailless" , "Predatory reptile (informal)" .)