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 "Bond, e.g.", "One who acts for another", "Broker", "Person acting on behalf of another", "He or she acts for actors", "Spy; representative", "One acting on behalf of another", "Rep", "Performer's manager" and "Representative - means".)