Severe piles? Sit differently! (8)
I believe the answer is:
pitiless
'severe' is the definition.
(thesaurus)
'piles? sit differently' is the wordplay.
'differently' indicates an anagram.
'piles'+'sit'='pilessit'
'pilessit' anagrammed gives 'PITILESS'.
(Other definitions for pitiless that I've seen before include "feeling nothing" , "Cold-hearted" , "Unfeeling" , "Harsh, cruel" , "Callous" .)