Keep quiet having reticence (8)
I believe the answer is:
preserve
'keep' is the definition.
(preserving is a kind of keeping)
'quiet having reticence' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'having' is a charade indicator (letters next to each other).
'reticence' becomes 'reserve' ('reserve' can be a synonym of 'reticence').
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "Keep in sugar" , "Prevent from deteriorating" , "defend" , "Maintain - jam" , "Particular interest" .)