Fool left in bed (4)
I believe the answer is:
clot
'fool' is the definition.
(I've seen this in other clues)
'left in bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'bed' becomes 'cot' (cot is a type of bed).
'l' put into 'cot' is 'CLOT'.
(Other definitions for clot that I've seen before include "(Of blood) solidify" , "Small lump of blood or other liquid" , "Fool - congeal" , "Coagulate - fool" , "3 [DUMMY]" .)