No...you need a weapon that is easier to hide better than the hidden blade. Yes, pun intended. A boot knife or one hidden in your sleeve is better, except that most of the time it won't escape a half decent search attempt.
Now, I devised first as concept and soon as real practice, a series of cheap, disposable and rather useful assassination weapons. Bear in mind on the word assassination weapons - they will be useless in actual combat.
First one depends on one thing really - the availability and use of poison in your game.
Ever read (or watched) Dune? Then you must be familiar with the Gom Jabbar - the poison needle ring from it. This is my variation. Its very simple, cheap, effective and disposable. Just approach someone, gently wrap your arm around his shoulders while pricking his neck and whispering "Poison" to his ear and you are done. You can hide the ring in a second, or just throw it away - its something small enough that won't be found immediately or at all.
But how its made?
Tools needed are two pairs of small pliers and a pair of scissors.
Material - plastic bottle, cheapest trift store 10 for a buck plastic and brass crap rings. Just make sure its from those that can be expanded or tightened with ease. Optional - aluminium foil.
Here's the full collection I made today.