Decry kettle coating used to create a protective barrier (4)

I believe the answer is:

dyke

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'barrier' is the definition.
(I know that dyke is a type of boundary)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dyke that I've seen before include "Anti-flood ditch/wall" , "Barrier to prevent flooding" , "Flood defence" , "Flood embankment" , "Ditch; earthwork" .)

I've seen this clue in The Irish Times.

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