Charter party once here beginning to embark on flowing sea (9)
I believe the answer is:
runnymede
'charter party once here' is the definition.
The definition suggests a singular noun which matches the answer.
'beginning to embark on flowing sea' is the wordplay.
'beginning to' indicates taking the first letters.
'on' means one lot of letters go next to another.
'flowing' becomes 'runny' (synonyms).
'sea' becomes 'med' (abbreviation of Mediterranean).
The first letter of 'embark' is 'e'.
'runny'+'med'='runnymed'
'e' put after 'runnymed' is 'RUNNYMEDE'.
Can you help me to learn more?
(Other definitions for runnymede that I've seen before include "Where King John signed Magna Carta in 1215" , "Democratic convention site" , "source of charter" , "where barons got the concession" , "Place in Surrey" .)