Rule out the hundred in cash (5)
I believe the answer is:
lucre
'cash' is the definition.
(I know that lucre is a type of cash)
'rule out the hundred in' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'the hundred' becomes 'C' (Roman numeral).
'in' means one lot of letters goes inside another.
'rule' is an anagram of 'lure'.
'lure' placed around 'c' is 'LUCRE'.
(Other definitions for lucre that I've seen before include "tin" , "There's money" , "Ulcer (anag)" , "(Filthy) profit" , "Sordid gain; riches" .)