Active fellow, he gets things done (5)
I believe the answer is:
Here is my best explanation:
'he gets things done' is the definition.
'active fellow' is the subsidiary indication.
'active' becomes 'a' (grammatical abbreviation).
'fellow' becomes 'gent' ('gent' can be a synonym of 'fellow'**).
(Other definitions for agent that I've seen before include "One representing others", "factor in Scotland", "One's representative", "Go-between", "Bond, e.g.", "Sales rep", "Actors' manager", "Actor", "Rep" and "One employed to incite others to commit illegal acts".)