Middleman regularly makes suggestion (4)
I believe the answer is:
idea
'suggestion' is the definition.
(I've seen this in another clue)
'middleman regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'middleman' are 'IDEA'.
'makes' is the link.
(Other definitions for idea that I've seen before include "Proposal" , "Inkling" , "Plato's Form is" , "Purpose" , "Suggestion" .)