Cruel and senseless to sing inside (8)
I believe the answer is:
inhumane
'cruel' is the definition.
('inhumane' can be similar in meaning to 'cruel')
'senseless to sing inside' is the wordplay.
'senseless' becomes 'inane' (I've seen this before).
'to sing' becomes 'hum' (humming is a kind of singing).
'inside' is an insertion indicator.
'inane' going around 'hum' is 'INHUMANE'.
'and' is the link.
(Other definitions for inhumane that I've seen before include "Not compassionate" , "Cruel" , "Heartless" , "Callous" .)