Pirate's flag (5,5)
I believe the answer is:
jolly roger
'pirate's flag' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for jolly roger that I've seen before include "Pirates' flag" , "Pirate flag" , "standard for boarders?" , "In the main a fearful sight" , "standard not welcomed in the main" .)