Fine letting on quietly (8)
I believe the answer is:
pleasing
'fine' is the definition.
(thesaurus)
'letting on quietly' is the wordplay.
'letting' becomes 'leasing' ('lease' can be a synonym of 'let').
'on' says to put letters next to each other.
'quietly' becomes 'p' (p for piano and quiet).
'leasing' after 'p' is 'PLEASING'.
(Other definitions for pleasing that I've seen before include "Satisfying or appealing" , "Delightful" , "agreeable - with 13d" , "Pleasurable; satisfying" , "Gratifying" .)