Write, as a secret message (6)
I believe the answer is:
encode
'write as a secret message' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for encode that I've seen before include "Turn message into secret symbols" , "Put (message) into ciphers" , "Put into secret symbols" , "Make cryptic" , "Encipher" .)