Assassin × Cinderella is a Manga/Manhwa/Manhua in english language, SUGGESTIVE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Neneko is spying on a rival organization's assassin, but her true identity is revealed, and the condition for her safety is "marriage"!? The world's sweetest and most dangerous «Contract Marriage» begins!!