Second-in-command (6)
I believe the answer is:
deputy
'second-in-command' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for deputy that I've seen before include "US sheriff's assistant" , "One given the power to act for another" , "Second-in-command" , "The next in command, replaces boss when necessary" , "Person appointed to substitute for another" .)