Crossword Genius

Mocking boy, one I expelled, that's heavily protected (8)

Ross

I believe the answer is:

ironclad

I'm a little stuck... Click here to teach me more about this clue!

'protected' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'mocking boy one i expelled that's heavily' is the wordplay.
I cannot quite understand how this works, but
'boy' could be 'ad' and 'ad' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for ironclad that I've seen before include "Early armoured warship" , "Protected as by armour - say an alibi" , "Absolutely determined" , "Inflexible" , "Well protected, like armoured alibi" .)

I've seen this clue in The Guardian.
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