Warning device's inventor (4)
I believe the answer is:
bell
I believe this clue is a triple definition!
'warning' is the first definition.
(a bell might be used as a warning)
'device's' is the second definition.
(bell is a kind of device)
'inventor' is the third definition.
(Alexander Graham Bell)
(Other definitions for bell that I've seen before include "Tinkler" , "It makes a ringing sound when hit with a clapper" , "End of oboe; inventor of telephone" , "Telephone call?" , "Boxing match signal" .)