Sister nursing case of cruel, painful back condition (9)
I believe the answer is:
sclerosis
'condition' is the definition.
(sclerosis is a kind of condition)
'sister nursing case of cruel painful back' is the wordplay.
'sister' becomes 'sis' (synonyms).
'nursing' is an insertion indicator.
'case of' suggests removing the centre (outsides of).
'painful' becomes 'sore' (similar in meaning).
'back' shows that the letters should be reversed in order.
'cruel' with its centre taken out is 'cl'.
'sore' back-to-front is 'eros'.
'cl'+'eros'='cleros'
'sis' placed around 'cleros' is 'SCLEROSIS'.
(Other definitions for sclerosis that I've seen before include "Hardening (of arteries etc) - cross isle (anag)" , "Hardening of body tissue" .)