Active fellow, he gets things done (5)
I believe the answer is:
agent
'he gets things done' is the definition.
'active fellow' is the wordplay.
'active' becomes 'a' (grammatical abbreviation).
'fellow' becomes 'gent' (I've seen this before**).
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "Instrument" , "One acting on behalf of another" , "He or she acts for actors" , "One representing others" , "Factor, spy" .)