Hello everyone,
a player of mine wants to create a new character and she wants to give
it the hobby "dancing".
Should we use an existing skill for it ? If yes, Martial Arts ?
Or Seduction ?
Or should we make up a new skill, "Dancing" based upon agility
or charisma ?
Does a solution exist in other rulebooks ? Do you have a better idea ?
Thanks in advance. 
The simplest solution is just call it "Info/Dancing". This would allow her to make info rolls to recognize important dance performers, identify styles/moves/steps/music types, contact a professional dance instructor, and in a pinch do "The Robot" without looking like a complete spaz.
However, you'd essentially be rolling Intelligence checks for all those, which kind of makes sense if you don't want to equate dancing with pure physical abilities. Even the most agile ninja might have two left feet when it comes to tripping the light fantastic, while a clumsy nerd might be able to tear him up on the dance floor.
However, it sounds like you might prefer something based on Dex/Agility or Charisma. In which case, I'd probably suggest looking at Bryant Durrell's "Entertainer" archetype and use the "Entertainment" skill, which is Charisma-based:
http://www.innocence.com/fengshui/house-rules/entertainer.html
(And let me just add that when Bryant Durrell, Dave Blewer, or David Eber's name is attached to something, you can darned near consider it Official Rules Type Stuff.)
Thanks alot Darrin, the linked "Entertainment"-skill looks like what
I've been looking for.
Btw.: Do you know what that means ? Spys and gamblers can't dance! 
lol.
Hmm. I suppose you could just say "Dancing" is part of "Seduction".
I was going to go with that one. Actually, maybe the relevant skill would depend on the kind of dancing, unless they've got Entertainer ...
Cutting a rug in dashing and romantic style could be Seduction, something complex where exact moves are required could be Info/Dancing and piling into the mosh pit would be closer to Martial Arts.