Feeling of sickness (6)
I believe the answer is:
nausea
'feeling of sickness' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nausea that I've seen before include "Feeling of imminent sickness or upset stomach" , "Feeling of imminent sickness, may be caused by sea" , "Malady" , "Queasy feeling" , "Sick feeling" .)