Small mat (7)
I believe the answer is:
coaster
'small mat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for coaster that I've seen before include "Small ship or beer mat" , "Mat for decanter or glasses" , "Small mat; cargo ship" , "Mat to protect a table" , "It goes under a schooner, perhaps" .)