A change involving one enclosure for birds (6)
I believe the answer is:
aviary
'enclosure for birds' is the definition.
(I've seen this before)
'a change involving one' is the wordplay.
'change' becomes 'vary' ('vary' can be a synonym of 'change').
'involving' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'vary' enclosing 'i' is 'viary'.
'a'+'viary'='AVIARY'
(Other definitions for aviary that I've seen before include "Bird enclosure" , "wildlife centre" , "Place where birds are kept" , "Large bird cage" , "Collection of birds" .)