Main barrier against encroachment (3,4)

I believe the answer is:

sea wall

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

'main barrier against' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'barrier' becomes 'wall' (a wall acts as a barrier).
'against' says to put letters next to each other.
'sea'+'wall'='SEA-WALL'

Can you help me to learn more?

(Other definitions for sea wall that I've seen before include "Land erosion protector" , "Coastal construction" , "Coastal barrier" , "Coastal defence" , "Mole, by water" .)

I've seen this clue in the Evening Standard.

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