Discrimination on increase - it makes it harder to listen (6)
I believe the answer is:
earwax
'it makes it harder to listen' is the definition.
The definition suggests a singular noun which matches the answer.
'discrimination on increase' is the wordplay.
'discrimination on' becomes 'ear' (I can't justify this - if you can you should believe this answer much more).
'increase' becomes 'wax' (waxing is a kind of increasing).
'ear'+'wax'='EARWAX'
Can you help me to learn more?
(Other definitions for earwax that I've seen before include "Hearing problem" , "Soft yellow secretion - cerumen" , "Shell-like secretion" , "Otic secretion" , "Substance accumulating in the lugholes" .)