Defence against main attack (7)

I believe the answer is:

seawall

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'attack' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'defence against main' is the wordplay.
'defence' becomes 'wall' (wall is a kind of defence).
'against' is a charade indicator (letters next to each other).
'main' becomes 'sea' (the main is an old name for the sea).
'wall' put after 'sea' is 'SEAWALL'.

Can you help me to learn more?

(Another definition for seawall that I've seen is " sea defence".)

I've seen this clue in The Irish Independent.

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