Bad back and radius affect vital organ (5)
I believe the answer is:
liver
'vital organ' is the definition.
(I've seen this before)
'bad back and radius' is the wordplay.
'bad' becomes 'evil' (evil is a kind of bad).
'back' says the letters should be written in reverse.
'and' means one lot of letters go next to another.
'radius' becomes 'r' (abbreviation in geometry).
'evil' written backwards gives 'live'.
'live'+'r'='LIVER'
'affect' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for liver that I've seen before include "Hepatic organ" , "Vital organ" , "Organ; reddish-brown" , "Bodily organ" , "Bile-secreting organ" .)