Melt together (4)
I believe the answer is:
fuse
'melt together' 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 "Meld" , "Unite by melting" , "Safety device in circuit" , "Melt; ignition device" , "'Blend, merge (4)'" .)