Quietly letting out ... Charming! (8)
I believe the answer is:
pleasing
'charming' is the definition.
(similar in meaning)
'quietly letting out' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'letting out' becomes 'leasing' (both can mean renting).
'p'+'leasing'='PLEASING'
(Other definitions for pleasing that I've seen before include "agreeable - with 13d" , "Delightful" , "making one happy" , "Gratifying" , "Pleasurable; satisfying" .)