Cruel and mostly senseless to restrict sound of activity (7)
I believe the answer is:
inhuman
'cruel' is the definition.
(inhuman can mean cruel or inhumane)
'mostly senseless to restrict sound of activity' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'senseless' becomes 'inane' (I've seen this before).
'to restrict' indicates putting letters inside (inserted letters are restricted or trapped).
'sound of activity' becomes 'hum' (eg the sound of a machine).
'inane' with its last letter taken off is 'inan'.
'inan' placed around 'hum' is 'INHUMAN'.
'and' acts as a link.
(Other definitions for inhuman that I've seen before include "Savage, brutal" , "Cruel, barbaric" , "Callously cruel" , "Beastly" , "Animal" .)