Crossword Genius

A thousand attack in armour (4)

Ross

I believe the answer is:

mail

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

'armour' is the definition.
(I know that mail is a type of body armor)

'a thousand attack' is the wordplay.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'attack' becomes 'ail' (I can't explain this - if you can you should believe this answer much more).
'm'+'ail'='MAIL'

'in' is the link.

Can you help me to learn more?

(Other definitions for mail that I've seen before include "Paper" , "Eg, letters" , "Letters and parcels" , "Posted letters" , "Letters - armour" .)

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