Sound of activity in university Her Majesty is going round (3)
I believe the answer is:
'sound of activity' is the definition.
(eg the sound of a machine)
'university her majesty is going round' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'her majesty' becomes 'hm' (short for Her Majesty).
'is going round' is an insertion indicator (some letters go around others).
'u' put within 'hm' is 'HUM'.
'in' is the link.
(Other definitions for hum that I've seen before include "odour" , "Sound of busy activity" , "Murmur" , "Reek" , "Insect sound of doubt" .)