Beer left in hospital department (3)
I believe the answer is:
ale
'beer' is the definition.
(type of beer)
'left in hospital department' is the wordplay.
'left' becomes 'L' (common abbreviation).
'in' means one lot of letters goes inside another.
'hospital department' becomes 'ae' (A and E, Accident and Emergency department).
'l' going inside 'ae' is 'ALE'.
(Other definitions for ale that I've seen before include "The A in CAMRA" , "English beer, may be bitter" , "One beer" , "Light-coloured beer" , "Brewery product" .)