Note bar contains unknown nut (6)
I believe the answer is:
brazil
'nut' is the definition.
(brazil is a kind of nut)
'note bar contains unknown' is the wordplay.
'note' becomes 'b' (musical note).
'bar' becomes 'rail' (rail is a kind of bar).
'contains' is an insertion indicator.
'unknown' becomes 'z' (represents an unknown value in algebra).
'rail' enclosing 'z' is 'razil'.
'b'+'razil'='BRAZIL'
(Other definitions for brazil that I've seen before include "Rio's country" , "Amazon's country" , "Country of Rio de Janeiro and Sao Paulo" , "Largest South American country" , "South American state" .)