They stand in the surf with boards underfoot (5)

I believe the answer is:

piers

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'they' is the definition.
Both the answer and definition are plural nouns.
Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for piers that I've seen before include "Landing stages" , "Old ploughman" , "Supports for bridge spans" , "Jetties" .)

I've seen this clue in The Independent.

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