Young player left in bed (4)
I believe the answer is:
colt
'young player' is the definition.
(young member of a sports team)
'left in bed' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'bed' becomes 'cot' (cot is a type of bed).
'l' inserted into 'cot' is 'COLT'.
(Other definitions for colt that I've seen before include "Young horse - gun" , "H [HORSE]" , "Young animal" , "Young stallion" , "Youngster" .)