Boat going over areas of still water (5)

I believe the answer is:

sloop

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'boat' is the definition.
(sloop is a kind of sailing vessel)

I don't understand how the rest of the clue works.

(Other definitions for sloop that I've seen before include "Single-masted sailing vessel" , "something that's rigged" , "one going to sea" , "Type of sailing boat" , "Sailing vessel with one mast" .)

I've seen this clue in The Irish Independent.

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