Firecracker part (4)
I believe the answer is:
fuse
'firecracker part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fuse that I've seen before include "Electrical fitting" , "Melt; ignition device" , "Join, coalesce" , "Common electrical safety device in homes etc" , "Join or blend to form an entity" .)