Museum wing, perhaps (5)
I believe the answer is:
annex
'museum wing perhaps' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for annex that I've seen before include "Join - add" , "Add by conquest or occupation" , "Attach after conquest" , "Add to the end; take possession of" , "Take (land) by conquest" .)