Fool left lying in bed (4)
I believe the answer is:
clot
'fool' is the definition.
(I've seen this before)
'left lying in bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'lying in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a kind of bed).
'l' put within 'cot' is 'CLOT'.
(Other definitions for clot that I've seen before include "Fool; solidified blood" , "Clumsy person" , "Stupid person - lump of blood" , "(Of blood) coagulate" , "Coagulate - fool" .)