Quietly say, 'Get clean clothes on!' (7)
I believe the answer is:
breathe
'quietly say' is the definition.
'breathe' can be an answer for 'say' (thesaurus). I'm not sure about the 'quietly' bit.
'clean clothes on' is the wordplay.
'clean' becomes 'bathe' (bathing is a kind of cleaning).
'clothes' means one lot of letters goes inside another (some letters clothe or cover others).
'on' becomes 're' ('on' can be mean 'with reference to').
'bathe' going around 're' is 'BREATHE'.
'get' is the link.
Can you help me to learn more?
(Other definitions for breathe that I've seen before include "Take into mouth" , "Take in 30" , "Inspire and expire air" , "Whisper" , "Do this before you go under" .)