Mounted apparatus to restrict noise (6)
I believe the answer is:
riding
'mounted' is the definition.
(both can mean on a horse)
'apparatus to restrict noise' is the wordplay.
'apparatus' becomes 'rig' (I've seen this before).
'to restrict' means one lot of letters goes inside another.
'noise' becomes 'din' (din is a kind of noise).
'rig' going around 'din' is 'RIDING'.
(Other definitions for riding that I've seen before include "On top" , "Canadian constituency" , "Parliamentary constituency (Canada) - former administrative district (Yorkshire)" , "Establishment teaching horsemanship" , "Going cycling, perhaps" .)