Lock’s opening with key? It’s for stocking material (5)
I believe the answer is:
lisle
'for stocking material' is the definition.
(I've seen this in another clue)
'lock's opening with key?' is the wordplay.
'opening' says to take the initial letters.
'with' means one lot of letters go next to another.
'key?' becomes 'isle' (I've seen this before).
The initial letter of 'locks' is 'l'.
'l'+'isle'='LISLE'
'it's' acts as a link.
(Other definitions for lisle that I've seen before include "Maker of hose" , "Type of cotton yarn" , "Fine cotton thread used in stockings" , "Strong tightly twisted cotton thread" , "Fine smooth cotton thread for stockings" .)