Softly cutting old bit of wire (6)
I believe the answer is:
staple
'bit of wire' is the definition.
(a staple is a piece of folded wire)
'softly cutting old' is the wordplay.
'softly' becomes 'p' (short for piano, musical term for softly).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'old' becomes 'stale' (synonyms).
'p' going into 'stale' is 'STAPLE'.
(Other definitions for staple that I've seen before include "Principle - metal fastener" , "Raw material" , "Paper-fastening device" , "Pleats in a paper pin" , "Main element" .)