Signal breaker, for example (4)
I believe the answer is:
wave
I believe this clue is a double definition.
'signal' is the first definition.
(thesaurus)
'breaker for example' is the second definition.
(breaker is a kind of wave)
(Other definitions for wave that I've seen before include "Billow" , "Hand signal; surge" , "Hand gesture - ridge of moving water" , "Drive forward" , "It may be permanent or Mexican" .)