One bird inhabiting swamps, primarily (4)
I believe the answer is:
ibis
'one bird inhabiting swamps primarily' is the definition.
I can't tell whether this defines the answer.
'one bird inhabiting swamps primarily' is the wordplay.
'one' becomes 'i' (Roman numeral).
'primarily' indicates taking the first letters.
The first letters of 'bird inhabiting swamps' is 'bis'.
'i'+'bis'='IBIS'
(Other definitions for ibis that I've seen before include "Bird with long neck and legs and a curved bill" , "State of central US, capital Lincoln" , "Wading bird sacred to the Egyptians" , "Wading bird of tropical waters" , "Large wading bird" .)