The most common and most likely answer to this crossword clue is the 5 plus 7 letter word NAVAL UNIFORM
N
A
V
A
L
U
N
I
F
O
R
M
Buddy explains!
The straight part
We know that "Correct co-ordinates, sailor assumes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case correct co-ordinates, sailor assumes = naval uniform.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.