Not thoroughly ill? (6)
I believe the answer is:
unwell
'ill?' is the definition.
(similar in meaning)
'not thoroughly' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'thoroughly' becomes 'well' (I've seen this in another clue).
'un'+'well'='UNWELL'
(Other definitions for unwell that I've seen before include "Somewhat ill" , "Ailing" , "Off-colour" , "Ill, not in good health" , "Sick, not in good health" .)