Join with a blowtorch (4)
I believe the answer is:
weld
'join with a blowtorch' 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 by fusion; dye plant" , "Fuse metal parts" , "Join (metals)" , "Join (metal) by melting" , "Bond" .)