Teens regularly autograph banner (6)
I believe the answer is:
ensign
'banner' is the definition.
(ensign is a type of flag)
'teens regularly autograph' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'autograph' becomes 'sign' (I've seen this before).
The alternate letters of 'teens' are 'en'.
'en'+'sign'='ENSIGN'
(Other definitions for ensign that I've seen before include "Military or naval flag" , "Look upon with contempt" , "Flag; rank" , "Lowly ranked officer" , "US naval rank" .)