Long tapering flag (6)
I believe the answer is:
pennon
'long tapering flag' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pennon that I've seen before include "one might be all of a flutter" , "Streamer on a lance" , "Long narrow triangular flag" , "tapered flag" , "Narrow, swallow-tailed standard" .)