Do you mean where his gym is or how you get to him once you're in his gym? Either way, I'll answer both.
He's behind the main city; go north and out of the city on a route like road, and then go right. His gym is past the new-Plasma HQ, where you can get N's Zorua.
Marriland guide - Driftveil chapter - watch the video and fast forward to around 23:40.
In the gym, this is how you get to Clay:
On the dotted lines, you will go under the pad, and you won't step on them/ have to challenge anyone there.
Courtesy of Marriland