A change involving one in birdcage (6)
I believe the answer is:
aviary
'birdcage' is the definition.
(I've seen this before)
'a change involving one' is the wordplay.
'change' becomes 'vary' (synonyms).
'involving' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'vary' placed around 'i' is 'viary'.
'a'+'viary'='AVIARY'
'in' acts as a link.
(Other definitions for aviary that I've seen before include "Where flights are limited" , "House for birds" , "chat room?" , "Place where birds are kept" , "Building for keeping birds" .)