Clear thinking may accept a purpose regarding sickness (8)
I believe the answer is:
nauseous
'clear thinking may accept a purpose regarding sickness' is the definition.
I can't judge whether this defines the answer.
'clear thinking may accept a purpose' is the wordplay.
'clear thinking' becomes 'nous' (I can't explain this - if you can you should give a lot more credence to this answer).
'may accept' indicates putting letters inside.
'purpose' becomes 'use' (synonyms).
'a'+'use'='ause'
'nous' placed around 'ause' is 'NAUSEOUS'.
Can you help me to learn more?
(Other definitions for nauseous that I've seen before include "Queasy, sick" , "Offensive to taste, smell" , "Affected with feeling of sickness" , "Feeling sensation of imminent sickness" , "Abhorrent" .)