One is caging black bird (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'one is caging black' is the wordplay.
'one' becomes 'i' (Roman numeral).
'caging' means one lot of letters goes inside another (I've seen this in other clues).
'black' becomes 'b' (abbreviation as in pencil classifications).
'i'+'is'='iis'
'iis' enclosing 'b' is 'IBIS'.
(Other definitions for ibis that I've seen before include "Retreat" , "Slowly discharge" , "may be sacred?" , "Bird with long neck and legs and a curved bill" , "seaside feeder?" .)