European in sauna developed sickness (6)
I believe the answer is:
nausea
'sickness' is the definition.
('nausea' can be a synonym of 'sickness')
'european in sauna developed' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'in' is an insertion indicator.
'developed' is an anagram indicator.
'sauna' is an anagram of 'nausa'.
'e' inserted inside 'nausa' is 'NAUSEA'.
(Other definitions for nausea that I've seen before include "Malady" , "Aversion" , "Use an 'a' (anag)" , "Sensation of queasiness" , "A feeling of disgust or illness" .)