Does anybody know the origin of the phrase 'clued up'?
etymonline has this hint about "to clue" as a verb
So the etymological "thread" is a follows
"clew" (Theseus thread provided by Ariadne) => "clue" => "to clue (someone)" => "to be clued".
There is also a different, but related, meaning in the nautical domain: [OED]
but in this case, the preferred spelling is "clew up".