Frivolous - can get switched on (5)

I believe the answer is:

light

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'frivolous' is the definition.
(light can mean trivial or unimportant)

I can't explain the rest of the clue.

(Other definitions for light that I've seen before include "Easy to carry" , "Easy to digest; flame" , "Frivolous -- insubstantial" , "guide in the dark" , "Airy; not heavy" .)

I've seen this clue in The Irish Independent.

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