It's a shade less useful on dull days (5)

I believe the answer is:

visor

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'it's a shade' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for visor that I've seen before include "Helmet's face-cover" , "Movable front part of helmet" , "Moveable piece of helmet" , "Means of saving face" , "One protecting" .)

I've seen this clue in the Evening Standard.

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