Sick feeling (6)
I believe the answer is:
nausea
'sick feeling' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nausea that I've seen before include "Feeling of revulsion" , "Aversion" , "Queasy feeling" , "Feeling of imminent sickness, may be caused by sea" , "Seasickness" .)