The most common and most likely answer to this crossword clue is the 2 plus 3 plus 4 letter word IN THE MAIN
I've seen this crossword clue in The Guardian
I
N
T
H
E
M
A
I
N
More clues leading to the result IN THE MAIN
Buddy explains!
The straight part
We know that "Generally/where the seas are high" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case generally/where the seas are high = in the main.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.