The most common and most likely answer to this crossword clue is the 8 letter word MERMAIDS
I've seen this crossword clue in Independent
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More clues leading to the result MERMAIDS
Buddy explains!

The straight part
We know that "females in sea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case females in sea = mermaids.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 MERMAIDS mean?
MERMAIDS noun- A mythological creature with a woman's head and upper body, and a tail of a fish.
- (as a modifier) Coloured a brilliant turquoise.
- A prostitute.