Almost all firms value clerkships because of the experience they offer associates and will therefore allow you to take one prior to starting full time (and hold your spot), regardless of whether it is a federal or state clerkship.
The one thing to be sensitive to, though, is that some firms are a little hesitant when it comes to clerkships because they are afraid that after having invested time and money training you during the summer, you will take the clerkship, boost your resume, and then apply to a firm elsewhere with your higher credentials, thus putting them out the amount of money they spent on you.
I would not tell the firm about your intentions to pursue a clerkship until you have an offer.