American supporting flag legend (4)
I believe the answer is:
saga
'legend' is the definition.
(thesaurus)
'american supporting flag' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'flag' becomes 'sag' ('sag' can be a synonym of 'flag').
'a' after 'sag' is 'SAGA'.
(Other definitions for saga that I've seen before include "Long detailed story" , "A long story or poem - it's a gas" , "Long heroic, family, story" , "Long heroic tale" , "Icelandic or Norse story" .)