Join a detonator (4)
I believe the answer is:
fuse
I believe this is a double definition.
'join' is the first definition.
(I know that join can be written as fuse)
'a detonator' is the second definition.
(I've seen this in another clue)
(Other definitions for fuse that I've seen before include "Mix together" , "Melt with heat" , "Safety device in circuit" , "Electrical fitting" , "Common electrical safety device in homes etc" .)