The most common and most likely answer to this crossword clue is the 11 letter word OCEAN-GOING
I've seen this crossword clue in The Guardian
O
C
E
A
N
-
G
O
I
N
G
Buddy explains!

The straight part
We know that "for long voyages" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case for long voyages = ocean-going.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 OCEAN-GOING mean?
OCEAN-GOING adjective- Designed and equipped to be capable of sailing on an ocean.