The most common and most likely answer to this crossword clue is the 13 letter word BATTLECRUISER
I've seen this crossword clue in The Guardian
B
A
T
T
L
E
C
R
U
I
S
E
R
Buddy explains!
The straight part
We know that "Large ship" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large ship = battlecruiser.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 BATTLECRUISER mean?
BATTLECRUISER noun- A type of warship at least as large as a battleship, and of greater speed, but with less armour and fewer guns. Introduced ca. 1908 and forming the leading squadrons of WWI battle fleets but obsolescent by WWII, having been replaced by the "fast battleship" type.
- In fiction, a warship of intermediate capability between cruisers and battleships