East Anglian town that's 'stumped' (stumped in time of prosperity) (6)
I believe the answer is:
boston
'east anglian town that's stumped' is the definition.
(town with a church nicknamed Boston Stump)
'stumped in time of prosperity' is the wordplay.
'stumped' becomes 'st' (cricket abbreviation).
'in' indicates putting letters inside.
'time of prosperity' becomes 'boon' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'st' inserted into 'boon' is 'BOSTON'.
(Other definitions for boston that I've seen before include "Mass housing this" , "US city of historic Tea Party" , "Card game; Lincolnshire town" , "Commonwealth capital" , "cards" .)