Quiet alto performed for Aristotle’s teacher (5)
I believe the answer is:
plato
'aristotle's teacher' is the definition.
(I've seen this in another clue)
'quiet alto performed' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'performed' indicates anagramming the letters.
'alto' anagrammed gives 'lato'.
'p'+'lato'='PLATO'
'for' acts as a link.
(Other definitions for plato that I've seen before include "Republic author" , "Teacher of Aristotle, pupil of Socrates" , "A plot (anag) (known to Aristotle?)" , "Philosopher of ancient Greece" , "Ancient Greek philosopher" .)