The most common and most likely answer to this crossword clue is the 4 plus 9 letter word DEAD RECKONING
I've seen this crossword clue in Financial Times
D
E
A
D
R
E
C
K
O
N
I
N
G
More clues leading to the result DEAD RECKONING
Buddy explains!
The straight part
We know that "How to establish one’s position" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how to establish one’s position = dead reckoning.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.