Mum's three sons touring public school (9)
I believe the answer is:
soundless
'mum's three' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
'sons touring public school' is the wordplay.
I cannot quite understand how this works, but
'sons' could be 'ss' (s is abbreviation for son) and 'ss' is located in the answer.
'school' could be 's' and 's' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for soundless that I've seen is " Silent".)