One illness after another (7)
I believe the answer is:
relapse
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'illness after another' is the wordplay.
'illness' becomes 'lapse' (I am not sure about this - if you are sure you should believe this answer much more).
'after' means one lot of letters go next to another.
'another' becomes 're' (I've seen this in other clues).
'lapse' after 're' is 'RELAPSE'.
Can you help me to learn more?
(Other definitions for relapse that I've seen before include "Fall back into sickness" , "unfortunate medical state" , "Worsen" , "Fall back into bad health" , "Repeals (anag.)" .)