Hi nodus1, I agree with BBM2 the answer looks as if it should be "cku" but if so it's neither the most robust nor cryptic possibility if you are looking for that clever "ah ha" moment.
Why not ask the excellent regular setters on the weekly "competitions" Clueless & Peer Review to treat it as such and parse (breakdown) their effort(s) allowing you to choose the most appropriate (I assume it has to be
"clue" clued for the plot. I'm sure the his of the ongoing
Peer Review, Les40, wouldn't mind you inserting an
"excuse-me" into the thread with a brief explanation and request to post submissions here.