Many include an alternative to being ill (6)
I believe the answer is:
poorly
'being ill' is the definition.
('poorly' can be similar in meaning to 'ill')
'many include an alternative' is the wordplay.
'many' becomes 'poly' (I can't explain this - if you can you should give a lot more credence to this answer).
'include' means one lot of letters goes inside another.
'an alternative' becomes 'or'.
'poly' going around 'or' is 'POORLY'.
'to' is the link.
Can you help me to learn more?
(Other definitions for poorly that I've seen before include "Ailing" , "In bad health" , "Not well" , "sick" , "Being unwell or deficient" .)