Yielding excellent reversal in illness (9)
I believe the answer is:
compliant
'yielding' is the definition.
(similar in meaning)
'excellent reversal in illness' is the wordplay.
'excellent' becomes 'ai' (resembles 'Ac', old-fashioned term for excellent).
'reversal' is a reversal indicator.
'in' means one lot of letters goes inside another.
'illness' becomes 'complnt' (I can't explain this - if you can you should believe this answer much more).
'ai' in reverse letter order is 'ia'.
'ia' placed into 'complnt' is 'COMPLIANT'.
Can you help me to learn more?
(Other definitions for compliant that I've seen before include "Submissive, acquiescent" , "Accommodating" , "Manageable" , "Subservient" , "Obliging" .)