European in mysterious sauna sickness (6)
I believe the answer is:
nausea
'sickness' is the definition.
(synonyms)
'european in mysterious sauna' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'in' indicates putting letters inside.
'mysterious' indicates an anagram.
'sauna' is an anagram of 'nausa'.
'e' inserted inside 'nausa' is 'NAUSEA'.
(Other definitions for nausea that I've seen before include "Condition of feeling imminent sickness" , "Sensation of queasiness" , "Ill-feeling" , "Malady" , "Queasy feeling" .)