In there as an intermediary (7)

I believe the answer is:

between

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'in' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I don't see?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for between that I've seen before include "Been wet (anag.)" , "needle, not the smallest"" , "Amongst" , "In the middle (of)" , "Flanked" .)

I've seen this clue in the Evening Standard.

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