Keep quiet and hold back (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep can be written as preserve)
'quiet and hold back' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'and' is a charade indicator (letters next to each other).
'hold back' becomes 'reserve' (reserve can mean to keep or hold something back).
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "Keep alive; jam" , "Keep from decay in perverse way" , "Jam or marmalade" , "Prevent from rotting" , "Prevent from deteriorating" .)