Crossword Genius

A ferrous alibi? (4-4,6)

Ross

I believe the answer is:

cast-iron excuse

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

'a ferrous alibi?' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'ferrous alibi?' is the wordplay.
'ferrous' becomes 'castiron' (I can't explain this - if you can you should believe this answer much more).
'alibi?' becomes 'excuse' (alibiing is a kind of excusing).
'castiron'+'excuse'='CAST-IRON EXCUSE'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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