Sing a bit of Schumann (3)
I believe the answer is:
'sing' is the definition.
(I know that hum is a more specific form of the action sing)
'a bit of schumann' is the wordplay.
'a bit of' indicates a hidden word.
'HUM' is hidden within 'schumann'.
(Other definitions for hum that I've seen before include "Sing like a bee?" , "Unpleasant smell" , "Sound (of business?)" , "Sing with closed lips" , "Sound of busy activity" .)