The majority of stupid guards make droning sound, being cruel (7)
I believe the answer is:
inhuman
'cruel' is the definition.
(I've seen this before)
'the majority of stupid guards make droning sound' is the wordplay.
'the majority of' means to remove the last letter (most but not all of the word is used).
'stupid' becomes 'inane' (I've seen this before).
'guards' indicates putting letters inside.
'make droning sound' becomes 'hum' (to hum is to make a droning noise).
'inane' with its final letter removed is 'inan'.
'inan' placed around 'hum' is 'INHUMAN'.
'being' acts as a link.
(Other definitions for inhuman that I've seen before include "Animal" , "Unfeelingly cruel" , "Nasty" , "Pitiless - barbarous" , "Without compunction" .)