the real thing and not a copy or something similar.

Cheap sparkling wines cannot be labelled 'champagne'. It has to be the real McCoy.

• a life-saver
• a lightning rod
• a likely story.
• a lingua franca
• a liquid lunch
• a list as long as arm
• a list as long as your arm
• a litmus test
• a little bird told me.
• a little bird told me so.
• a little horror
• a little tin god
• alive and well
• a live wire
• a living death
• a living hell
• all along
• all along the line
• all at once
• all but
• all but something
• all ears
• all eyes
• all eyes are on
• all eyes are on someone