English, irrational medic, spreading plague (8)
I believe the answer is:
epidemic
'plague' is the definition.
(I've seen this before)
'english irrational medic spreading' is the wordplay.
'english' becomes 'E' (abbreviation).
'irrational' becomes 'pi' (an irrational number).
'spreading' indicates anagramming the letters.
'medic' with letters rearranged gives 'demic'.
'e'+'pi'+'demic'='EPIDEMIC'
(Other definitions for epidemic that I've seen before include "lot of disease" , "Citrus fruit" , "Widespread outbreak of an infectious disease" , "Fast food item" , "Widespread infection" .)