That queasy feeling (6)
I believe the answer is:
nausea
'that queasy feeling' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nausea that I've seen before include "Sensation of queasiness" , "Feeling of imminent sickness or upset stomach" , "Seasickness" , "Feeling of revulsion" , "Squeamishness" .)