Something which is sadly declining in the modern world.
To have a clue implies that you will apply common sense to a problem, or at least RTFM, before saying or doing something stupid. Having a clue instantly disqualifies you from being a MarkeTroid.
(I disagree: is a technician turned salesperson necessarily a marketroid? They can have Clue... -- 
CraigBox)
Also a 1970's movie, based around the board game of the same name.
3 pages link to Clue:
