The most common and most likely answer to this crossword clue is the 9 letter word NONVERBAL
I've seen this crossword clue in The Daily Telegraph
N
O
N
V
E
R
B
A
L
More clues leading to the result NONVERBAL
Buddy explains!
The straight part
We know that "using signals to communicate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case using signals to communicate = nonverbal.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 NONVERBAL mean?
NONVERBAL noun- A sound, gesture, etc. that is not a word.
- (of communication) in a form other than written or spoken words, such as gestures, facial expressions or body language.
- (grammar, somewhat rare) of or relating to a word that belongs to any part of speech other than that of verbs
- Unable to speak.