Standard Texan GIs need to maintain if upset
I believe the answer is:
ensign
'standard' is the definition.
(both can mean a flag)
'texan gis need to maintain if upset' is the wordplay.
'to maintain' indicates the answer is hidden within the clue (the solution is kept or maintained by the clue).
'if upset' shows that the letters should be reversed in order (in a down clue, letters go up).
'ngisne' is hidden within 'texan gis need'.
'ngisne' reversed gives 'ENSIGN'.
(Other definitions for ensign that I've seen before include "Badge, flag" , "SECOND PART OF 10" , "In British infantry, a colours bearer" , "US naval rank" , "Ship's colours" .)