Standard message understood on trip (5,5)
I believe the answer is:
jolly roger
'standard' is the definition.
(I know that Jolly Roger is a type of standard)
'message understood on trip' is the wordplay.
'message understood' becomes 'roger' ('roger that').
'on' is a charade indicator (letters next to each other).
'trip' becomes 'jolly' (I've seen this before).
'roger' put after 'jolly' is 'JOLLY ROGER'.
(Other definitions for jolly roger that I've seen before include "standard not welcomed in the main" , "Pirates' flag" , "Pirate flag" , "standard for boarders?" , "In the main a fearful sight" .)