In there as an intermediary (7)
I believe the answer is:
'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" .)