I have pranged my cross tube and it is aluminium, just a nick though...can i still ride it?

Question

it's not the straight tube, not the down tube, but the tube that runs crosswise. the prang is 5mm long, 2mm dent.

Answer 1

depends what you use the bike for, if you do cross country on it it may bend or break

Answer 2

What is the girth of the cross tube? Going on the measurements you supplied, the structural integrity of the bike is most probably fine and safe to ride. However to be safe you should get it checked out at a bicycle repair shop.