Seclusion is, to a lion, wretched (9)
I believe the answer is:
isolation
'seclusion' is the definition.
(I've seen this before)
'is to a lion wretched' is the wordplay.
'wretched' is an anagram indicator.
'to'+'a'+'lion'='toalion'
'toalion' anagrammed gives 'olation'.
'is'+'olation'='ISOLATION'
(Other definitions for isolation that I've seen before include "Staff in such a hospital working with" , "State of separation, closing off" , "Cutting off" , "Quarantine; solitude" , "Remote and lonely state" .)