Step across hard filament (6)
I believe the answer is:
thread
'filament' is the definition.
(I've seen this before)
'step across hard' is the wordplay.
'step' becomes 'tread' ('tread' can be a synonym of 'step'**).
'across' indicates putting letters inside.
'hard' becomes 'h'.
'tread' placed around 'h' is 'THREAD'.
(Other definitions for thread that I've seen before include "Strand, cord" , "Prominent spiral part of a screw" , "Strand for sewing" , "Essential element" , "What sewer takes" .)