European in sauna cured sickness (6)
I believe the answer is:
nausea
'sickness' is the definition.
(synonyms)
'european in sauna cured' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'in' indicates putting letters inside.
'cured' is an anagram indicator.
'sauna' with letters rearranged gives 'nausa'.
'e' inserted within 'nausa' is 'NAUSEA'.
(Other definitions for nausea that I've seen before include "Sensation of queasiness" , "Queasy feeling" , "Sick feeling" , "Seasickness" , "Aversion" .)