Philosopher quietly changes a lot (5)
I believe the answer is:
plato
'philosopher' is the definition.
(I know that Plato is a philosopher)
'quietly changes a lot' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'changes' is an anagram indicator.
'a'+'lot'='alot'
'alot' with letters rearranged gives 'lato'.
'p'+'lato'='PLATO'
(Other definitions for plato that I've seen before include "Greek character" , "To pal up with Greek philosopher" , "Ancient Athenian philosopher, pupil of Socrates" , "A plot (anag) (known to Aristotle?)" , "Philosopher of ancient Greece" .)