Dummy left in baby's bed (4)
I believe the answer is:
clot
'dummy' is the definition.
(I've seen this before)
'left in baby's bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'baby's bed' becomes 'cot' (cot is a kind of baby's bed).
'l' going within 'cot' is 'CLOT'.
(Other definitions for clot that I've seen before include "3 [DUMMY]" , "Thick lump - thick person" , "(Of blood) coagulate" , "Stupid person - lump of blood" , "Fool; solidified blood" .)