It simply provides cover (5,8)

I believe the answer is:

plain envelope

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'it' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'simply provides cover' is the wordplay.
I cannot quite understand how this works, but
'simply' could be 'plain' (I've seen this before) and 'plain' is present in the answer.
'cover' could be 'envelope' (I've seen this before) and 'envelope' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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