Cruel coalmine I bar (8)
I believe the answer is:
pitiless
'cruel' is the definition.
(I've seen this before)
'coalmine i bar' is the wordplay.
'coalmine' becomes 'pit' (I've seen this before).
'bar' becomes 'less' (both can mean 'not including').
'pit'+'i'+'less'='PITILESS'
(Other definitions for pitiless that I've seen before include "Remorseless, ruthless" , "Callous" , "Unrelenting" , "Harsh, cruel" , "Unfeeling" .)