A letter to interpret for money (5)
I believe the answer is:
bread
'money' is the definition.
(bread is slang for money)
'a letter to interpret' is the wordplay.
'a letter' becomes 'b' ().
'to interpret' becomes 'read' (reading is a kind of interpreting).
'b'+'read'='BREAD'
'for' is the link.
(Other definitions for bread that I've seen before include "Staff of life" , "''Eaten . . . . . is soon forgotte''" , "Staple diet" , "Foodstuff; money" , "Gracefully thin" .)