Two unknowns cut perfect instrument (9)
I believe the answer is:
xylophone
'instrument' is the definition.
(xylophone is a kind of instrument)
'two unknowns cut perfect' is the wordplay.
'two unknowns' becomes 'XY' (algebra).
'cut' becomes 'lop' (lopping is a kind of cutting).
'perfect' becomes 'hone' (synonyms).
'xy'+'lop'+'hone'='XYLOPHONE'
(Other definitions for xylophone that I've seen before include "Oxyphenol (anag.)" , "Percussion instrument with wooden bars" , "Patrick Moore's instrument" , "Musical instrument with bars" , "Instrument with wooden bars to be struck" .)