Pistol left in bed (4)
I believe the answer is:
colt
'pistol' is the definition.
(I've seen this before)
'left in bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'bed' becomes 'cot' (cot is a kind of bed).
'l' inserted inside 'cot' is 'COLT'.
(Other definitions for colt that I've seen before include "Type of American revolver" , "member of the youth team" , "Young stallion" , "firearm" , "runner" .)