A good PVA woodglue that’s waterproof like http://www.titebond.com/product/glues/e8d40b45-0ab3-49f7-8a9c-b53970f736af available at most hardware stores or online everywhere.

Lift gently and get the whole surface area saturated if you can. Wipe it down with a wet paper towl to clean off excess. Clamping looks impossible here, so pick a weight that concentrates itself along the broken piece. Leave it alone until dry - usually 24 hours min. Sand until smooth.

Edit: just saw that it has a screw in it. Lift the screw out maybe half an inch to assure more glue coverage and crank it back down. Still use the weight on the unsecured piece.