Notes unknown hat (3)
I believe the answer is:
Here is my best explanation:
'hat' is the definition.
(fez is a kind of hat)
'notes unknown' is the subsidiary indication.
'notes' becomes 'fe'.
'unknown' becomes 'z' (algebra**).
(Other definitions for fez that I've seen before include "Tarboosh", "Turkish tasselled cap", "Felt cap", "Felt hat", "Red cap with tassel worn by Muslim", "Egyptian tasseled hat", "Cap with tassel", "Eastern felt cap with a tassel", "High flat-topped red cap with tassel worn by some Muslims" and "Brimless conical cap".)