Not well (3)
'not well' is the definition.
('ill' is the opposite to 'well')
This is all the clue.
(Other definitions for ill that I've seen before include "state, briefly" , "US state" , "Unhealthy" , "Faulty" , "Harmful" .)
'not well' is the definition.
(I know that not well can be written as poorly)
This is the entire clue.
(Other definitions for poorly that I've seen before include "In bad health" , "Out of sorts" , "sick" , "in a bad way" , "Being unwell or deficient" .)