Go-between, tidy earner, I'm excited (12)
I believe the answer is:
Here is my best explanation:
'go-between' is the definition.
('intermediary' can be a synonym of 'go-between')
'tidy earner i'm excited' is the subsidiary indication.
'excited' indicates anagramming the letters.
'tidyearnerim' anagrammed gives 'intermediary'.
(Other definitions for intermediary that I've seen before include "Go-between" and "In between".)