Melt into one (4)
I believe the answer is:
fuse
'melt into one' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for fuse that I've seen before include "Unite by melting" , "Tube or cord for igniting an explosive" , "Safeguard in electrical circuit" , "Mix together" , "Weld together through heat" .)