The most common and most likely answer to this crossword clue is the 16 letter word DIRECTION-FINDER
I've seen this crossword clue in The Times
D
I
R
E
C
T
I
O
N
-
F
I
N
D
E
R
Buddy explains!
The straight part
We know that "navigational aid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case navigational aid = direction-finder.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.