Step across hot filament (6)
I believe the answer is:
thread
'filament' is the definition.
(I've seen this before)
'step across hot' is the wordplay.
'step' becomes 'tread' (tread is a kind of step).
'across' indicates putting letters inside.
'hot' becomes 'h' (abbreviation, eg on taps).
'tread' placed around 'h' is 'THREAD'.
(Other definitions for thread that I've seen before include "Subject" , "The rib going around a screw" , "Messages on a theme" , "Length of spun cotton or wool" , "Sewing chord" .)