Incomplete hump sure to return in an arm bone (7)
I believe the answer is:
humerus
'an arm bone' is the definition.
(humerus is a kind of arm bone)
'incomplete hump sure to return' is the wordplay.
'incomplete' means to remove the last letter.
'to return' is a reversal indicator.
'sure' written backwards gives 'erus'.
'hump' with its last letter taken away is 'hum'.
'hum'+'erus'='HUMERUS'
'in' acts as a link.
(Other definitions for humerus that I've seen before include "Upper arm bone" , "Arm bone - the "funny bone"" , "Arm bone (that sounds comical)" , "Bone from shoulder to elbow" , "Boneo of the upper arm" .)