They may be worn by those shortening sails (7)

I believe the answer is:

reefers

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'sails' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe there's a link between them I don't understand?

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

Can you help me to learn more?

(Other definitions for reefers that I've seen before include "Illegal cigarettes" , "Joints (sl.)" , "Sailors' jackets" , "Jackets; joints" , "Nautical jackets" .)

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

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