Run after penniless middleman (6)
I believe the answer is:
broker
'middleman' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for broker that I've seen before include "One acts as an agent for others in buying and selling" , "See 7" , "Agent paid by commission" , "Negotiate with" , "Middleman, dealer" .)