Coat shared by a couple? (6)

I believe the answer is:

enamel

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'coat' is the definition.
(I know that enamel is a type of paint)

I don't understand the rest of the clue.

(Other definitions for enamel that I've seen before include "hard stuff" , "Hard shiny coating on surface" , "Smooth hard paint" , "Decorative inlay" , "protective coat" .)

I've seen this clue in the Evening Standard.

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