Postgraduate quietly makes diagram (3)
I believe the answer is:
map
'diagram' is the definition.
(I've seen this in another clue)
'postgraduate quietly' is the wordplay.
'postgraduate' becomes 'ma' (Master of Arts is a postgraduate degree).
'quietly' becomes 'p' (p for piano and quiet).
'ma'+'p'='MAP'
'makes' acts as a link.
(Other definitions for map that I've seen before include "Diagram of an area" , "(Make a) geographical representation" , "Sketch" , "Travel guide" , "Chart" .)