Scramble as GCHQ in vault beneath Eton's walls? (7)
I believe the answer is:
encrypt
'scramble as gchq' is the definition.
I can't judge whether this definition defines the answer.
'vault beneath eton's walls?' is the wordplay.
'vault' becomes 'crypt' (I've seen this in another clue).
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'walls?' suggests removing the centre (I've seen 'walls' mean this (the walls are the letters on each edge)).
'eton' with its middle removed is 'en'.
'crypt' put after 'en' is 'ENCRYPT'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for encrypt that I've seen before include "Obscure words of" , "Change into coded version" , "Put (a message) into code" , "Encipher" , "Cipher" .)