Alias letters (3)
I believe the answer is:
'alias letters' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aka that I've seen before include "Notation with initials indicating alternative name" , "Notation indicating alternative identity" , "Notation indicating alias or other name" , "Using the byline" , "Also called (abbrev.)" .)