The most common and most likely answer to this crossword clue is the 7 letter word PELAGIC
I've seen this crossword clue in The Guardian
P
E
L
A
G
I
C
More clues leading to the result PELAGIC
Buddy explains!

The straight part
We know that "of the open seas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of the open seas = pelagic.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 PELAGIC mean?
PELAGIC noun- Any organism that lives in the open sea rather than in coastal or inland waters.
- Living in the open sea rather than in coastal or inland waters.
- Of or pertaining to oceans.