Old country with aviation centre (5)
I believe the answer is:
zaire
'old country' is the definition.
'with aviation centre' is the wordplay.
'with' indicates putting letters inside.
'aviation' becomes 'air' (synonyms).
'centre' becomes 'ze' (I am not sure about this - if you are sure you should believe this answer much more).
'air' placed into 'ze' is 'ZAIRE'.
Can you help me to learn more?
(Other definitions for zaire that I've seen before include "Previous name of African state, achieved independence from Belgium in 1960" , "renamed African area" , "Previous name of Congo" , "Former name of DRC" , "Country, formerly" .)