Long flag (8)
(Other definitions for streamer that I've seen before include "Long flag or decoration" , "Ribbon - bad cold?" , "Long flag, tapering perhaps" , "Banner headline; long strip" , "Banner, flag; fishing fly" .)
'long flag' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pennon that I've seen before include "Streamer on a lance" , "Small triangular flag" , "Narrow, swallow-tailed standard" , "tapered flag" , "Flag carried by 9 across" .)
(Other definitions for pennant that I've seen before include "Triangular flag" , "Small flag" , "Tapering flag" , "Standard" , "Banner" .)