My MacBook Pro 15.4" (A1286 - Late 2008) trackpad is missing clicks and generally not working I notice it when I'm highlighting text as well. Halfway through the a selection it will unselect. It only seems to click right at the very bottom now.

Do I need to replace or clean it?
The battery might be swelling and pushing up against the bottom of the trackpad. If this is the case, as soon as you remove the battery it will work as expected (assuming no permanent damage has been done).

Replace Immediately! Swollen batteries can be dangerous.