Do you sound soft when you put it on? (7)
I believe the answer is:
muffler
'do' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'sound soft when you put it on?' is the wordplay.
I cannot really understand how this works, but
'sound' could be 'muffle' (I have seen 'Suppress sound ' mean 'muffle' so perhaps 'sound' could also mean 'muffle') and 'muffle' is located in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'you' is the link.
Can you help me to learn more?
(Other definitions for muffler that I've seen before include "Warming garment" , "Wrap or scarf worn round the neck" , "Thick scarf - silencer (US)" , "Kind of scarf" , "Sound deadener" .)