The most common and most likely answer to this crossword clue is the 8 letter word MERCATOR
I've seen this crossword clue in Independent
M
E
R
C
A
T
O
R
More clues leading to the result MERCATOR
- Not entirely happy about animal round projector
- Map-maker finding flashy car on a hill
- Design came to include river and peak, as cartographer
- French sea boat provided with men, one improving marine navigation
- Man who mapped strange moon crater without working
- Map’s original creator with new layout
- Geographer in vehicle on a high rocky hill
- Mapmaker‘s car on a hill
- Cartographer in vehicle, on schedule coming back
- Geographer to put aboard French sea vehicle
Buddy explains!
The straight part
We know that "one improving marine navigation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one improving marine navigation = mercator.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.