Young one left in bed (4)
I believe the answer is:
colt
'young' is the definition.
(colt is a kind of young)
'one left in bed' is the wordplay.
'one left' becomes 'L' (common abbreviation).
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a type of bed).
'l' going into 'cot' is 'COLT'.
(Other definitions for colt that I've seen before include "firearm" , "Male foal" , "Youngster" , "Young stallion" , "Young Arab maybe" .)