Oliver's principal ironclad opponent (3)
I believe the answer is:
foe
'opponent' is the definition.
(I know that foe is a type of opponent)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for foe that I've seen before include "Antagonist" , "Evil rival" , "Am archaic enemy" , "Opponent" , "opposing side" .)