One flat on the bed? (4)

I believe the answer is:

sole

I'm a little stuck... Click here to teach me more about this clue!

'one' is the definition.
(synonyms)

I don't understand the remainder of the clue.

(Other definitions for sole that I've seen before include "Kind of fish - lose it" , "fish in the sea!" , "Dover - -" , "Exclusive class of fish" , "A lone flat fish?" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.