Exenna returned to a subsidiary building (6)
I believe the answer is:
annexe
'subsidiary building' is the definition.
'annexe' can be an answer for 'building' (I've seen this before). I'm not certain of the 'subsidiary' bit.
'exenna returned' is the wordplay.
'returned' says the letters should be written in reverse.
'exenna' backwards is 'ANNEXE'.
'to a' is the link.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Conservatory, e.g." , "Appropriate into one's territory" , "Addendum" , "Added section of document" , "An addition to a document or building" .)