Wanted poster letters (3)
I believe the answer is:
aka
'wanted poster 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 "Also known as, initially" , "Also called (abbrev.)" , "Notation indicating alias or other name" , "Notation indicating alternative identity" , "Using the byline" .)