Clams might not be able to take up all DOC's a protein skimmer can but they are extremely efficient nitrate reducers. They quite literaly suck them up out of the water at a high rate.
One clam might not make much difference, depending on size off-course, but several can do a good job.
Dr.Mac and other have used filters comprising of multiple clams as nitrate filters.
In commercial farming operations they even go so far as to introduce nitrates into the system to provide food for these fascinating animals.
So I guess the answer to your question is yes and no. It depends on A LOT of factors such as your bio load, amount and species of clams used, your feeding habits, etc.