Wading bird, one initially breeding in swamps (4)
I believe the answer is:
ibis
'wading bird' is the definition.
(ibis is a kind of wading bird)
'one initially breeding in swamps' is the wordplay.
'one' becomes 'i' (Roman numeral).
'initially' suggests taking the first letters.
The initial letters of 'breeding in swamps' is 'bis'.
'i'+'bis'='IBIS'
(Other definitions for ibis that I've seen before include "One has large bill" , "Hotel group" , "Bird, sacred to ancient Egyptians" , "Large wader" , "Wading bird of warm regions" .)