Luxuriant merriment during second date, money spent on wicked things (5,4)
I believe the answer is:
slush fund
'money spent on wicked things' is the definition.
'luxuriant merriment during second date' is the wordplay.
'luxuriant' becomes 'lush' (synonyms).
'merriment' becomes 'fun' (synonyms).
'during' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'lush'+'fun'='lushfun'
's'+'d'='sd'
'lushfun' going inside 'sd' is 'SLUSH FUND'.
(Other definitions for slush fund that I've seen before include "Money to be used for financing corruption" , "Sum of money to be used for bribery" , "Reserve of money used for bribery" , "Questionable political capital" .)