Prepared to interpret a letter (5)
I believe the answer is:
ready
'prepared' is the definition.
(associated in meaning)
'interpret a letter' is the wordplay.
'interpret' becomes 'read' (reading is a kind of interpreting).
'a letter' becomes 'y' (Y is a letter of the alphabet).
'read'+'y'='READY'
'to' is the link.
(Other definitions for ready that I've seen before include "Prepare < dough" , "rough companion" , "Primed" , "Fully-prepared" , "Willing (to do something), prepare" .)