Render unreadable WSJ Crossword Clue

Think about how messages are sent secretly, like a spy's hidden note.

?????? means to convert information into a different form, especially for the purpose of secrecy or security.

The clue 'Render unreadable' suggests transforming text into a form that cannot be easily understood, which is what encoding does. Encoding converts information into a different format, often for secure transmission, making it unreadable to those without the proper key or decoding method.

Disguise, in a way, Make secret, in a way, Prepare for transmission, perhaps, Convert to cipher, Make hard to read, in a way