Many a lie having been found out, is antagonistic (7)
I believe the answer is:
hostile
'antagonistic' is the definition.
(similar in meaning)
'many a lie' is the wordplay.
'many' becomes 'host' (I've seen this before).
'a lie' becomes 'ile' (I can't justify this - if you can you should give a lot more credence to this answer).
'host'+'ile'='HOSTILE'
'having been found out is' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for hostile that I've seen before include "Unkind" , "Bellicose" , "Enemy" , "Aggressive" , "Showing angry opposition" .)