Wing sounds like an unknown quantity (6)
I believe the answer is:
annexe
'wing' is the definition.
(synonyms)
'sounds like an unknown quantity' is the wordplay.
'sounds like' becomes 'anne' (I am not sure about this - if you are sure you should believe this answer much more).
'an unknown' becomes 'x' (algebra).
'quantity' becomes 'e' (e is a kind of quantity).
'anne'+'x'+'e'='ANNEXE'
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Supplementary part" , "Building joined to a larger one" , "Wing of the main building" , "Additional room etc for house" , "Side building" .)