Cream light source is preserved (7)
I believe the answer is:
pickled
'is preserved' is the definition.
(similar in meaning)
'cream light source' is the wordplay.
'cream' becomes 'pick' (both can mean the best of a group).
'light source' becomes 'led' (abbreviation for Light Emitting Diode).
'pick'+'led'='PICKLED'
(Other definitions for pickled that I've seen before include "Preserved - drunk" , "Preserved in liquid" , "Brahms and Liszt" , "Blotto" , "Drunk (as a gherkin?)" .)