Quarantine, to a lion, is criminal (9)
I believe the answer is:
isolation
'quarantine' is the definition.
(thesaurus)
'to a lion is criminal' is the wordplay.
'criminal' is an anagram indicator.
'to'+'a'+'lion'+'is'='toalionis'
'toalionis' with letters rearranged gives 'ISOLATION'.
(Other definitions for isolation that I've seen before include "Man could display this" , "State of separation, closing off" , "Quarantine; solitude" , "Detachment" , "loneliness" .)