Charlie left in bed (4)
I believe the answer is:
clot
'charlie' is the definition.
(both can mean a foolish person)
'left in bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a kind of bed).
'l' inserted into 'cot' is 'CLOT'.
(Other definitions for clot that I've seen before include "Fool; solidified blood" , "Coagulate - fool" , "Thick lump - thick person" , "A lump in a blood vessel" , "3 [DUMMY]" .)