Identify bird in swamp, initially, as wader (4)
I believe the answer is:
ibis
'wader' is the definition.
(ibis is a kind of wader)
'identify bird in swamp initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'identify bird in swamp' is 'IBIS'.
'as' acts as a link.
(Other definitions for ibis that I've seen before include "Sacred Egyptian bird" , "Wader once sacred in Egypt" , "seaside feeder?" , "Bird, sacred to ancient Egyptians" , "Eastern" .)