Impermeable (10)

I believe the answer is:

waterproof

'impermeable' is the definition.
(synonyms)

This is all the clue.

(Other definitions for waterproof that I've seen before include "Rain-resistant" , "Impermeable" , "Impervious to rain" , "Protective garment" , "Mac" .)

I've seen this clue in The Sunday Times.

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