Bone throb turning steady (7)
I believe the answer is:
humerus
'bone' is the definition.
(humerus is a kind of bone)
'throb turning steady' is the wordplay.
'throb' becomes 'hum' (both can mean a regular noise).
'turning' says the letters should be written in reverse.
'steady' becomes 'sure' (similar in meaning).
'sure' in reverse letter order is 'erus'.
'hum'+'erus'='HUMERUS'
(Other definitions for humerus that I've seen before include "Bone from shoulder to elbow" , "Upper arm bone" , "Upper-arm bone" , "What's often thought funny" , "Boneo of the upper arm" .)