One flat on the bed? (4)
I believe the answer is:
sole
'one' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(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?" .)