Believe there are letters to revise (6)
I believe the answer is:
credit
'believe there' is the definition.
'credit' can be an answer for 'believe' (I've seen this before). I am not certain of the 'there' bit.
'letters to revise' is the wordplay.
'letters' becomes 'cr' (I am not sure about this - if you are sure you should believe this answer much more).
'to revise' becomes 'edit' (I've seen this before).
'cr'+'edit'='CREDIT'
'are' acts as a link.
Can you help me to learn more?
(Other definitions for credit that I've seen before include "it's on the cards?" , "(Account) sum received" , "Praise; tick" , "Belief - trust - favourable acknowledgment" , "Believe; good reputation" .)