Fuse metal with a blowtorch (4)
I believe the answer is:
weld
'fuse metal with a blowtorch' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for weld that I've seen before include "Solder" , "Unite metals by heating to melting point" , "Join; fuse" , "Join together; dye plant" , "Join by fusion; dye plant" .)