Bone crusher ultimately in ferment (7)
I believe the answer is:
stirrup
'bone' is the definition.
(bone of the ear)
'crusher ultimately in ferment' is the wordplay.
'ultimately' suggests the final letters.
'in' indicates putting letters inside.
'ferment' becomes 'stir up' (both can mean to provoke or excite).
The last letter of 'crusher' is 'r'.
'r' inserted inside 'stirup' is 'STIRRUP'.
(Other definitions for stirrup that I've seen before include "one puts the boot into" , "Rider's foot rest" , "Horse rider's footrest" , "Equine foot rest" , "Equestrian footrest" .)