If confidence low, the Exchequer ultimately produces not so many (5)
I believe the answer is:
fewer
'not so many' is the definition.
('few' can be the opposite of 'many' so perhaps 'fewer' is reasonable for 'not so many')
'if confidence low the exchequer ultimately' is the wordplay.
'ultimately' suggests the final letters.
The final letters of 'if confidence low the exchequer' are 'FEWER'.
'produces' is the link.
(Other definitions for fewer that I've seen before include "in smaller numbers" , "smaller number of people" , "Reduced" , "Less in number" , "Not so many" .)