Rampant great penning dime novel (8)
I believe the answer is:
epidemic
'rampant' is the definition.
(thesaurus)
'great penning dime novel' is the wordplay.
'great' becomes 'epic'.
'penning' means one lot of letters goes inside another.
'novel' is an anagram indicator.
'dime' anagrammed gives 'demi'.
'epic' placed around 'demi' is 'EPIDEMIC'.
(Other definitions for epidemic that I've seen before include "Prevalent disease" , "Disease affecting many in a population at the same time" , "Rash" , "Citrus fruit" , "Fast food item" .)