You say you are certain inside, his rates are extortionate? (6)
I believe the answer is:
usurer
'his rates are extortionate?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'you say you are certain' is the wordplay.
'you say' shows a homophone (sound like).
'are certain' becomes 'surer' (I am not sure about this - if you are sure you should believe this answer much more).
'you' is a homophone of 'u'.
'u'+'surer'='USURER'
'inside' is the link.
Can you help me to learn more?
(Other definitions for usurer that I've seen before include "One takes interest" , "One who charges excessive interest" , "Person taking a lot of interest" , "Moneylender who charges very high interest" , "One lending money at unreasonable rates" .)