Material? This is not so (13)
I believe the answer is:
inconsequence
'material? this' is the definition.
The definition suggests a singular noun which matches the answer.
'not so' is the wordplay.
I cannot really understand how this works, but
'not' could be 'con' (I've seen this in other clues) and 'con' is located in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for inconsequence that I've seen before include "Lack of importance" , "As a result" , "Lack of logic" .)