Pitiless on being confined in pen (5)
I believe the answer is:
stony
'pitiless' is the definition.
(thesaurus)
'on being confined in pen' is the wordplay.
'being confined in' is an insertion indicator.
'pen' becomes 'sty' (sty is a kind of pen).
'on' put inside 'sty' is 'STONY'.
(Other definitions for stony that I've seen before include "Rock-strewn" , "made of marble?" , "Cold, unfeeling" , "penniless" , "Rocky" .)