System of signalling with the arms (9)
I believe the answer is:
semaphore
'system of signalling with the arms' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for semaphore that I've seen before include "See arm hop in signalling system" , "Way to signal" , "Old flag signalling system" , "Strangely, home as per a system of communication" , "System of signalling with the arms" .)