Nautical hazards mean one takes in sail (5)

I believe the answer is:

reefs

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'nautical hazards' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for reefs that I've seen before include "Groups of submerged rock" , "coral banks" , "Ridges of rock or sand beneath sea surface" , "Submerged ridges of rock or coral near the surface of water" .)

I've seen this clue in The Globe and Mail.

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