Notes unknown hat (3)
I believe the answer is:
fez
'hat' is the definition.
(fez is a kind of hat)
'notes unknown' is the wordplay.
'notes' becomes 'fe' ().
'unknown' becomes 'z' (algebra).
'fe'+'z'='FEZ'
(Other definitions for fez that I've seen before include "Egyptian felt cap" , "Egyptian tasseled hat" , "Flat-topped red cap with tassel worn by Muslim men" , "Tarboosh" , "Tasselled red cap" .)