The most common and most likely answer to this crossword clue is the 8 letter word STAYSAIL
I've seen this crossword clue in The Guardian
S
T
A
Y
S
A
I
L
Buddy explains!

The straight part
We know that "a flapper at sea/a flapper at sea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a flapper at sea/a flapper at sea = staysail.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does STAYSAIL mean?
STAYSAIL noun- A fore-and-aft rigged sail whose luff can be affixed to a stay running forward from a mast to the deck, the bowsprit or to another mast.