The most common and most likely answer to this crossword clue is the 9 letter word SIGNALMAN
I've seen this crossword clue in The Times
S
I
G
N
A
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M
A
N
Buddy explains!

The straight part
We know that "naval transmitter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case naval transmitter = signalman.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 SIGNALMAN mean?
SIGNALMAN noun- Somebody employed to operate the signals and points of a railway.
- A member of the armed forces responsible for signalling.
- Somebody employed to direct rigging or crane operations by providing a different point of view.