Support commanding officer in post (6)
I believe the answer is:
second
'support' is the definition.
(eg to second an idea is to support it)
'commanding officer in post' is the wordplay.
'commanding officer' becomes 'co'.
'in' means one lot of letters goes inside another.
'post' becomes 'send' (synonyms).
'co' placed inside 'send' is 'SECOND'.
(Other definitions for second that I've seen before include "Short time; attendant at a duel" , "jiffy" , "Moment; back" , "bit" , "Mo" .)