Pitiless, relentless (11)
I believe the answer is:
remorseless
This clue is a double definition.
'pitiless' is the first definition.
(I know that pitiless can be written as remorseless)
'relentless' is the second definition.
(I've seen this before)
(Other definitions for remorseless that I've seen before include "Lacking compassion" , "Cruel; without respite" , "Unmerciful" , "Not letting up," , "Unrelenting" .)