US officer shows the flag (6)
I believe the answer is:
ensign
This clue is a double definition.
'us officer' is the first definition.
(rank in some US armed forces)
'the flag' is the second definition.
(ensign is a kind of flag)
'shows' is the link.
(Other definitions for ensign that I've seen before include "Ship's flag to show nationality" , "In British infantry, a colours bearer" , "Soldier; standard" , "Flag indicating allegiance" , "National flag" .)