Hi, I haven't fished Clark's in awhile, but I used to quite abit. I just wanted to pass on a few things, 'cause some folks seem to think Clarks is kinda tough.
the fish get a little spooky when the water is low, but they are well accustomed to anglers and they are generally hungry. Wade slowly, wear camo , but don't go to crazy trying to "hide" from the fish. I do best just quietly shuffling through the pools and keeping my casts "short" 20-25. I do well with 5 o 6x tippet and general purpose dark attractors like an Adams oral Renegade. Try terrestrials also.
If you get frustrated, try an inchworm. And around dark, put on a green weenie.
Clarks is a pretty stream with "some" wild fish. those are usually caught above the fly-stretch. there are also a few big fish, but compared to the Cumberland Valley Streams nearby, they are very rare. Good Luck!
Padraic