Get an organising middleman (5)
I believe the answer is:
agent
'middleman' is the definition.
(thesaurus)
'get an organising' is the wordplay.
'organising' is an anagram indicator.
'get'+'an'='getan'
'getan' anagrammed gives 'AGENT'.
(Other definitions for agent that I've seen before include "One's representative" , "One employed to incite others to commit illegal acts" , "Traveller" , "Substance that exerts some force or effect" , "Person who does business on behalf of another" .)