Ordering pie, medic revealing widespread occurrence of disease (8)
I believe the answer is:
epidemic
'widespread occurrence of disease' is the definition.
(I've seen this before)
'ordering pie medic revealing' is the wordplay.
'ordering' is an anagram indicator.
'revealing' indicates anagramming the letters.
'pie' anagrammed gives 'epi'.
'medic' anagrammed gives 'demic'.
'epi'+'demic'='EPIDEMIC'
(Other definitions for epidemic that I've seen before include "Fast food item" , "Dim piece (anag)" , "medical emergency" , "Widespread outbreak of an infectious disease" , "eruption" .)