Repetitive disease spares one small shrub (8)
I believe the answer is:
berberis
'shrub' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'repetitive disease spares one small' is the wordplay.
'repetitive disease spares' becomes 'berber' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'I' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'berber'+'i'+'s'='BERBERIS'
Can you help me to learn more?
(Other definitions for berberis that I've seen before include "plant" , "N African Muslim" , "Spiny shrub sometimes planted to deter burglars" , "This bush be briers, strangely" , "Shrub made from beer and ribs" .)