The same is observed in large-scale outbreak (8)
I believe the answer is:
epidemic
'outbreak' is the definition.
(I know that outbreak can be written as epidemic)
'the same is observed in large-scale' is the wordplay.
'the same' becomes 'idem' (Latin for 'the same', used in citations).
'is observed in' means one lot of letters goes inside another.
'large-scale' becomes 'epic' (I've seen this before).
'idem' put inside 'epic' is 'EPIDEMIC'.
(Other definitions for epidemic that I've seen before include "Prevalent disease" , "Outbreak of disease which spreads rapidly" , "Disease affecting many in a population at the same time" , "medical emergency" , "Disease affecting many people simultaneously" .)