Write note about flag (6)
I believe the answer is:
pennon
'flag' is the definition.
(I know that pennon is a type of flag)
'write note about' is the wordplay.
'write' becomes 'pen' (synonyms).
'note' becomes 'n' (abbreviation).
'about' becomes 'on' (as in a book 'on' a certain topic).
'pen'+'n'+'on'='PENNON'
(Other definitions for pennon that I've seen before include "Flag carried by 9 across" , "Long narrow triangular flag" , "Knight's personal flag" , "one might be all of a flutter" , "Streamer on a lance" .)