The most common and most likely answer to this crossword clue is the 6 plus 8 letter word FLYING DUTCHMAN
I've seen this crossword clue in The Times
F
L
Y
I
N
G
D
U
T
C
H
M
A
N
More clues leading to the result FLYING DUTCHMAN
Buddy explains!
The straight part
We know that "an ominous sight, in the main" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an ominous sight, in the main = flying dutchman.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.