Terrible nature of legitimacy abandoned by leader (9)
I believe the answer is:
awfulness
'abandoned by leader' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'terrible nature of legitimacy' is the wordplay.
'terrible' becomes 'awful' (synonyms).
'nature of legitimacy' becomes 'ness' (I can't justify this - if you can you should give a lot more credence to this answer).
'awful'+'ness'='AWFULNESS'
Can you help me to learn more?
(Another definition for awfulness that I've seen is " Quality of being horrid".)