Drone - boring, senseless, cruel (8)
I believe the answer is:
inhumane
'cruel' is the definition.
(similar in meaning)
'drone boring senseless' is the wordplay.
'drone' becomes 'hum' (I've seen this before).
'boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'senseless' becomes 'inane' (I've seen this before).
'hum' inserted inside 'inane' is 'INHUMANE'.
(Other definitions for inhumane that I've seen before include "Cruel" , "Callous" , "Not compassionate" , "Heartless" .)