Create an 'active' class for partial url?
There is a module for that - Menu Trail By Path

Menu Trail By Path sets the active-trail on menu items according to the current url.

For example if you are at yoursite.com/blog/category1/article1

Menu Items with these paths will get the active-trail class on them and expand accordingly.

blog
blog/category1
blog/category1/article1


This is particularly useful if you want a lot of nodes to appear as children of certain nodes / taxonomy term / views / referenced nodes / etc, but do not want to add them all to the menu. eg. hundreds of blog articles.

Menu Trail By Path is best used in conjunction with Pathauto.