Such a letter could be read aloud! (3)
I believe the answer is:
red
'such a letter could' is the definition.
I can't tell whether this definition defines the answer.
'read aloud' is the wordplay.
'aloud' indicates a 'sounds like' (homophone) clue.
'read' sounds like 'RED'.
'be' acts as a link.
Can you help me to learn more?
(Other definitions for red that I've seen before include "Merchant flag" , "'If you see it, perhaps you're angry (3)'" , "Colour of communism" , "- Cross; - carpet" , "Warning light colour" .)