Twin's quite unknown (4)
I believe the answer is:
ally
'twin's' is the definition.
(thesaurus)
'quite unknown' is the wordplay.
'quite' becomes 'all' (I've seen this before).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "A friendly and reliable country" , "Cooperating country" , "One co-operating with another" , "Join by marriage, treaty etc" , "Join - friend" .)