Switch rotated on explosive device (9)
I believe the answer is:
detonator
'explosive device' is the definition.
(detonator is a kind of explosive device)
'switch rotated on' is the wordplay.
'switch' is an anagram indicator.
'rotated'+'on'='rotatedon'
'rotatedon' with letters rearranged gives 'DETONATOR'.
(Other definitions for detonator that I've seen before include "cap" , "One sets off bang" , "small charge" , "Explosion starter" , "this will cause an outburst" .)