Improve a financial centre's dishonesty (9)
I believe the answer is:
mendacity
'dishonesty' is the definition.
(mendacity is a kind of dishonesty)
'improve a financial centre's' is the wordplay.
'improve' becomes 'mend' (mending is a kind of improving).
'financial centre' becomes 'city'.
'mend'+'a'+'city'='MENDACITY'
(Other definitions for mendacity that I've seen before include "Quality or habit of telling lies or being deceitful" , "Untruthfulness" , "Lying" .)