i doubt that will do anything to help a clam recover.
the opening of the shell is provided by the tension of a ligament. as noted in that post. when the clam is at rest, it is open. tying it closed would only serve to restrict mantel extension and limit it's ability to photosynthesize.
a month isn't a long time for a clam. it could have been having issues long before it came to you, and just now started to succumb to them. in my experience Tridacna clams (especially T. derasa) look fine right up until they're not, and telling a healthy specimen from a doomed specimen takes some experience, and a little bit of luck.
the first thing i always asses with a new clam is the growth edge on the shell. that's a good lagging indicator for long term health.
here is a picture of mine early morning, so his mantle was out of the way. you can see near the top of the shell the clean, white edge, indicating recent calcium precipitation on the shell"
next is the reaction to stimuli, mainly shading. to be clear, a clam can have good reaction to shading, but still be in trouble. when the clam's reaction is weak or not present, that's usually an indicator of imminent demise.
after that i like to just generally look at it. the mantle should have a vibrant hue, and it should just look right for the species (not a good description, i know, but it's the best way i can describe it). the siphon shouldn't be gaping, on T. derasa it should be a fairly closed intake siphon.
is this the first clam you've tried to keep?
is it possible that you have something in your tank picking on it?
were there any signs of physical damage to the mantle or foot? any snails present?