Put your bike on a stand so you can inspect it and find the source of the clanking. Grab the right crankarm and push it in & out towards the frame and feel for play. Repeat for the other side. Spin the crank slowly and listen to what makes noise. Look closely in that area and home in on the source. See if its dependent upon the gear you're using. Something may be hitting something else.

If you've got damaged crankarms from riding it previously when it was loose, most likely there's absolutely nothing the shoppe can do to fix it for you. You'll need to buy a new crankarm.