Prepared to scan a letter (5)
I believe the answer is:
ready
'prepared' is the definition.
(similar in meaning)
'scan a letter' is the wordplay.
'scan' 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 "bread" , "All set to go" , "Fully prepared" , "Primed" , "Intelligent" .)