Keep quiet, hold back (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep can be written as preserve)
'quiet hold back' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'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 "area where game is held" , "Keep safe or in unchanged condition" , "Area of activity" , "Jam or marmalade" , "Keep alive, maintain" .)