Bone in back that I’m unsure about (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'back that i'm unsure about' is the wordplay.
'back' becomes 'stern' (stern is a kind of back).
'that i'm unsure' becomes 'mu' (I am not sure about this - if you are sure you should believe this answer much more).
'about' shows that the letters should be reversed in order.
'mu' written backwards gives 'um'.
'stern'+'um'='STERNUM'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "Munster (anag.)" , "Chest bone" , "front of breast" , "somewhere in chest" , "big bone" .)