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