Crossword Genius

Vessel that may be covered by another, e.g. schooner (7)

Ross

I believe the answer is:

coaster

'vessel' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for coaster that I've seen before include "Small table mat" , "Trading vessel" , "on which some mug may be made to stand?" , "Mat for decanter or glasses" , "Mat to protect a table" .)

I've seen this clue in The Independent.
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