Essentially gazump wealthy financial centre (6)
I believe the answer is:
zurich
'financial centre' is the definition.
(Zurich is an important financial city)
'essentially gazump wealthy' is the wordplay.
'essentially' says to take the centre.
'wealthy' becomes 'rich' (similar in meaning).
The central letters of 'gazump' are 'zu'.
'zu'+'rich'='ZURICH'
(Other definitions for zurich that I've seen before include "Swiss city noted for gnomes" , "Swiss banking centre" , "See 8" , "Swiss financial centre" , "Largest Swiss city" .)