A French duke brought in twenty pounds for a rascal (9)
I believe the answer is:
scoundrel
'a rascal' is the definition.
(thesaurus)
'a french duke brought in twenty pounds' is the wordplay.
'a french' becomes 'un' ('a' in French).
'duke' becomes 'd'.
'brought in' means one lot of letters goes inside another.
'twenty' becomes 'score' ('score' can be a synonym of 'twenty').
'pounds' becomes 'l' (L can represent the monetary pound).
'un'+'d'='und'
'und' put into 'score' is 'scoundre'.
'scoundre'+'l'='SCOUNDREL'
'for' is the link.
(Other definitions for scoundrel that I've seen before include "Rat" , "Untrustworthy figure" , "Disreputable type" , "Villain" , "Bounder" .)