Queasy feeling (6)
I believe the answer is:
nausea
'queasy feeling' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nausea that I've seen before include "A feeling of disgust or illness" , "Squeamishness" , "Unpleasant feeling" , "Feeling of sickness or disgust" , "Condition of feeling imminent sickness" .)