Level of tone not softly played is an irritation (4)
I believe the answer is:
itch
'irritation' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for itch that I've seen before include "Hunger, longing" , "Prickling" , "Irritating sensation on skin" , "Nagging wish" , "A nagging desire" .)