Area to change enclosing one zoo feature (6)
I believe the answer is:
aviary
'zoo feature' is the definition.
(I've seen this in another clue)
'area to change enclosing one' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'to change' becomes 'vary' (synonyms).
'enclosing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'vary' enclosing 'i' is 'viary'.
'a'+'viary'='AVIARY'
(Other definitions for aviary that I've seen before include "Cut gem; small film part" , "section of zoo" , "Bird house" , "wildlife centre" , "Bird sanctuary" .)