Severity - a quality of the back part? (9)
I believe the answer is:
sternness
'severity' is the definition.
(I know that sternness is a type of severity)
'back part?' is the wordplay.
'back' becomes 'stern' (stern is a kind of back).
'part?' becomes 'ness' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stern'+'ness'='STERNNESS'
'a quality of the' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sternness that I've seen before include "severity" , "serious nature!" , "Grimly serious quality" .)