The most common and most likely answer to this crossword clue is the 4 letter word NAVY
I've seen this crossword clue in The Guardian
N
A
V
Y
More clues leading to the result NAVY
- Unknown vehicle's back for service
- Unknown vehicle reversing is blue, close to Oxford
- Service digger wanting third
- Dark blue vehicle reversing at back of alley
- Dark blue Bible displayed in major city
- Service vehicle lifted onto end of jetty
- Labourer‘s only one against Kind of Blue?
- Tone reflects section of The Lady Vanishes
- Service vehicle going around yard
- Vote against including Victory in the fleet
Buddy explains!

The straight part
We know that "Tone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tone = navy.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 NAVY mean?
NAVY noun- A country's entire sea force, including ships and personnel.
- A governmental department in charge of a country's sea force.
- A dark blue colour, usually called navy blue.
- Having the dark blue colour of navy blue.
- Belonging to the navy; typical of the navy.