Harsh exile or ban possibly (10)
I believe the answer is:
inexorable
'harsh' is the definition.
(I've seen this before)
'exile or ban possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'exile'+'or'+'ban'='exileorban'
'exileorban' is an anagram of 'INEXORABLE'.
(Other definitions for inexorable that I've seen before include "Relentless" , "Impossible to stop" , "Unstoppable" , "Inescapable" , "Harsh" .)