One flustering young woman once (7)
I believe the answer is:
flapper
I believe this is a double definition.
'one flustering' is the first definition.
'young woman once' is the second definition.
(Other definitions for flapper that I've seen before include "girl in 20s" , "Unconventional nineteen twenties girl" , "She was a flighty young thing" , "Fashionable young woman of the 1920s" , "one likely to panic?" .)