Slightly unwell -- or, on reflection, extremely poorly (4)
I believe the answer is:
ropy
'slightly unwell' is the definition.
(I've seen this before)
'or on reflection extremely poorly' is the wordplay.
'on reflection' says the letters should be written backwards.
'extremely' suggests removing the centre.
'poorly' with its middle removed is 'py'.
'or' reversed gives 'ro'.
'ro'+'py'='ROPY'
(Other definitions for ropy that I've seen before include "Slightly off-colour" , "Substandard" , "Slightly unwell" , "like some painters?" , "Of poor quality (informal)" .)