Having a use in an upset, or sickness (6)
I believe the answer is:
nausea
'sickness' is the definition.
('nausea' can be a synonym of 'sickness')
'having a use in an upset' is the wordplay.
'having' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'upset' says the letters should be written backwards.
'an' backwards is 'na'.
'a'+'use'='ause'
'ause' going within 'na' is 'NAUSEA'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for nausea that I've seen before include "Sensation of queasiness" , "Squeamishness" , "Feeling of sickness or disgust" , "Ill-feeling" , "A feeling of disgust or illness" .)