Detonator (4)
I believe the answer is:
fuse
'detonator' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fuse that I've seen before include "Melt; ignition device" , "Electrical fitting" , "Tube or cord for igniting an explosive" , "Melt with heat" , "'Blend, merge (4)'" .)