Charter party once here beginning to embark on flowing sea (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

My last in, bemused by the definition and perhaps by 1ac and thinking works outing. Flowing is RUNNY, the sea you want is the MED, finish with Embark’s beginning. The charter is the Great one, signed (some say sealed) at a quarter past noon.