It's common sense there's a purpose in feeling sick (8)
I believe the answer is:
nauseous
'feeling sick' is the definition.
(I've seen this before)
'it's common sense there's a purpose in' is the wordplay.
'it's common sense' becomes 'nous' (nous is a kind of common sense).
'there's' says to put letters next to each other.
'purpose' becomes 'use' ('use' can be a synonym of 'purpose'**).
'in' indicates putting letters inside.
'a' put next to 'use' is 'ause'.
'nous' enclosing 'ause' is 'NAUSEOUS'.
(Other definitions for nauseous that I've seen before include "Offensive to taste, smell" , "Affected with feeling of sickness" , "Sick-making" , "Causing feeling of sickness or disgust" , "Afflicted with feeling of imminent sickness" .)