Separation, to a lion, is upsetting (9)
I believe the answer is:
isolation
'separation' is the definition.
(isolation is a kind of separation)
'to a lion is upsetting' is the wordplay.
'upsetting' indicates anagramming the letters.
'to'+'a'+'lion'+'is'='toalionis'
'toalionis' is an anagram of 'ISOLATION'.
(Other definitions for isolation that I've seen before include "Detachment" , "Man could display this" , "keeping apart" , "Separation - solitude" , "Being separated" .)