Exactly 50% of disease in one area, part of Europe (6)
I believe the answer is:
iberia
'part of europe' is the definition.
(peninsula including Spain and Portugal)
'exactly 50 of disease in one area' is the wordplay.
'exactly 50 of disease' becomes 'beri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'area' becomes 'a' (maths abbreviation).
'i'+'a'='ia'
'beri' inserted inside 'ia' is 'IBERIA'.
Can you help me to learn more?
(Other definitions for iberia that I've seen before include "Mediterranean peninsula" , "European region" , "SW European peninsula" , "Spain and Portugal - airline" , "Peninsula that was Roman province" .)