European stops terrible sauna sickness (6)
I believe the answer is:
nausea
'sickness' is the definition.
(I know that sickness can be written as nausea)
'european stops terrible sauna' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'stops' is an insertion indicator (stopping can mean blocking a gap).
'terrible' indicates an anagram.
'sauna' is an anagram of 'nausa'.
'e' placed into 'nausa' is 'NAUSEA'.
(Other definitions for nausea that I've seen before include "Ill-feeling" , "Sickness, biliousness" , "Seasickness" , "Sensation of queasiness" , "Unpleasant feeling" .)