Quietly stops old joiner (6)
I believe the answer is:
staple
'joiner' is the definition.
(something which joins)
'quietly stops old' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'stops' means one lot of letters goes inside another.
'old' becomes 'stale' ('stale' can be a synonym of 'old'**).
'p' inserted inside 'stale' is 'STAPLE'.
(Other definitions for staple that I've seen before include "Paper securer; main" , "Paper pin - basic" , "Fundamental" , "Fastener - principal" , "important things to eat" .)