Fuse with heat (4)
I believe the answer is:
weld
'fuse with heat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for weld that I've seen before include "Join (metal) by melting" , "Lewd (anag.)" , "Join by fusion; dye plant" , "Join; fuse" , "Join using fusion" .)