Yes you can eat the clam siphon since the clam siphon is 100 percent edible although it's not very easy for some people to chew or swallow.
The clam siphon is edible and can be eaten but it's rubbery and tough, especially compared to the tender body of the steamer clam.
I like eating clams and tried eating the clam siphon before but it was too rubbery and tough for me to chew or eat so I threw it out.
But if you want to eat it and you have good teeth and can chew the clam siphon you can certainly eat it.
You can eat clams raw or cooked but I prefer to eat the clams cooked.