Vast sum Nero wasted: nothing saved (8)
I believe the answer is:
enormous
'vast' is the definition.
(I've seen this before)
'sum nero wasted nothing saved' is the wordplay.
'wasted' indicates an anagram.
'nothing' becomes 'O' (O resembles 0).
'saved' is an insertion indicator (save can mean to keep hold of).
'sum'+'nero'='sumnero'
'sumnero' with letters rearranged gives 'enrmous'.
'enrmous' placed around 'o' is 'ENORMOUS'.
(Other definitions for enormous that I've seen before include "Extremely large" , "Fat" , "Vast" , "Gross" , "Immense" .)