The best energy-saving light given longer life (7)
I believe the answer is:
pickled
'longer life' is the definition.
I can't tell whether this defines the answer.
'the best energy-saving light given' is the wordplay.
'the best' becomes 'pick' (the best possible pick or selection).
'energy' becomes 'e' (physics symbol as in E=mcd).
'saving' means one lot of letters goes inside another (inserted letters are kept or saved).
'light given' becomes 'ld' (I can't explain this - if you can you should give a lot more credence to this answer).
'e' placed within 'ld' is 'led'.
'pick'+'led'='PICKLED'
Can you help me to learn more?
(Other definitions for pickled that I've seen before include "Preserved - drunk" , "Soused" , "Drunk (as a gherkin?)" , "Blotto" , "Preserved in liquid" .)