Is topless, rich and amusing (duke kept back money for immoral purposes) (5,4)
I believe the answer is:
slush fund
'money for immoral purposes' is the definition.
(a slush fund is used to bribe people)
'is topless rich and amusing duke' is the wordplay.
'topless' means to remove the first letter.
'rich' becomes 'lush' (similar in meaning).
'and' says to put letters next to each other.
'amusing' becomes 'fun' (synonyms).
'duke' becomes 'd' (abbreviation).
'is' with its first letter removed is 's'.
's'+'lush'+'fun'+'d'='SLUSH FUND'
'kept back' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for slush fund that I've seen before include "Questionable political capital" , "Money to be used for financing corruption" , "Sum of money to be used for bribery" , "Reserve of money used for bribery" .)