Rash half contained by large-scale ... (8)
I believe the answer is:
epidemic
'rash' is the definition.
(I've seen this before)
'half contained by large-scale' is the wordplay.
'half' becomes 'demi' (prefix as in 'demigod').
'contained by' indicates putting letters inside.
'large-scale' becomes 'epic' (I've seen this before).
'demi' going within 'epic' is 'EPIDEMIC'.
(Other definitions for epidemic that I've seen before include "health emergency" , "Rash" , "Unfortunately widespread" , "Widespread infection" , "lot of disease" .)