A device easily seen through (4)

I believe the answer is:

lens

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'a device' is the definition.
(lens is a kind of device)

I don't understand the remainder of the clue.

(Other definitions for lens that I've seen before include "One could project a picture" , "town in France" , "Part of photographer's equipment" , "Transparent optical devices" , "Piece of glass in optical instrument" .)

I've seen this clue in the Evening Standard.

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