Middleman regularly revealing plan (4)
I believe the answer is:
idea
'plan' is the definition.
(thesaurus)
'middleman regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'middleman' are 'IDEA'.
'revealing' acts as a link.
(Other definitions for idea that I've seen before include "Aim" , "Creative spark" , "Clue" , "Thought, belief" , "Flash of inspiration?" .)