The tough quality that can put an edge on things (5)
I believe the answer is:
steel
'the tough quality that' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'put an edge on things' is the wordplay.
'put' becomes 'sit' (sitting is a kind of putting).
'an edge' suggests removing the centre (I've seen 'edging' mean this (edging means the letters on the edges)).
'on things' becomes 'eel' (I am not sure about this - if you are sure you should believe this answer much more).
'sit' with its centre taken out is 'st'.
'st'+'eel'='STEEL'
'can' acts as a link.
Can you help me to learn more?
(Other definitions for steel that I've seen before include "brace" , "Form of iron" , "Make hard and unfeeling" , "May be cold" , "Knife-sharpener" .)