Available to interpret a letter (5)
I believe the answer is:
ready
'available' is the definition.
(similar in meaning)
'interpret a letter' is the wordplay.
'interpret' becomes 'read' (synonyms).
'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 "Willing (to do something), prepare" , "Primed" , "Cash; within reach" , "Prompt and adequate" , "Fully-prepared" .)