Almost speak up, wretch: you a traitor too? (2,2,5)
I believe the answer is:
et tu brute
'you a traitor too?' is the definition.
I know nothing about this answer so I can't judge whether this works.
'almost speak up wretch' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'speak' becomes 'utter' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'wretch' becomes 'brute' (I can't explain this - if you can you should believe this answer much more).
'utter' with its last letter taken off is 'utte'.
'utte' written backwards gives 'ettu'.
'ettu'+'brute'='ET TU BRUTE?'
Can you help me to learn more?
(Other definitions for et tu brute that I've seen before include "Shakespeare's version of Julius Caesar's last words, in Latin" , "According to the Bard, Caesar's final words" , "Julius Caesar's supposed last words" , "You've let me down" , "Caesar's reported words on being stabbed" .)