the best and safest thing to use to take anything sticky off your bike would be to use denatured
Alcohol.

 

If you stated that the sticker is underneath the clearcoat, you would have to basically repaint the tank with all that is involved. First, you would have to sand the clearcoat off the tank, remove the sticker, repaint the basecoat then apply clearcoat. You could always just sand it down, reprimer, wetsand, basecoat, then clear coat.

Options are limited on the tank unless you plan on completely repainting.