An officer and in command (7)
I believe the answer is:
mandate
'command' is the definition.
(I've seen this before)
'an officer and in' is the wordplay.
'an officer' becomes 'mate' (mate is a kind of officer).
'in' is an insertion indicator.
'mate' going around 'and' is 'MANDATE'.
(Other definitions for mandate that I've seen before include "Written authority" , "Authority conferred on a tandem" , "Authority for action" , "Authority to act" , "Authority to do something" .)