Light left on before service (7)
I believe the answer is:
lantern
'light' is the definition.
(lantern is a kind of light)
'left on before service' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'before' becomes 'ante' (I've seen this before**).
'service' becomes 'rn' (abbreviation for Royal Navy).
'l'+'ante'+'rn'='LANTERN'
(Other definitions for lantern that I've seen before include "outside D [DECORATION]" , "Light in a protective casing" , "Light in a transparent protective case" , "Transparent case for holding a light" , "It illuminates" .)