Ruthless chap avoiding pained expression (4)
I believe the answer is:
fell
'ruthless' is the definition.
(fell can mean deadly or ruthless)
I can't explain the remainder of the clue.
(Other definitions for fell that I've seen before include "Cause to tumble down < hill" , "As Jack came down" , "Brutal -- high moorland" , "Mountain - lost footing - deadly" , "Came to pass" .)