Shipwreck Singer: When can the first ability be activated?
"Attacks this turn if able" means: when the Attack phase comes around, if that creature is able to attack, it must. That doesn't come with an obligation to leave it able to attack, though, and your opponent may rightly wish to do something that leaves that creature unable to attack.

Your opponent can use Manaweft's Sliver ability. Then, when the Attack phase comes around, Manaweft Sliver won't be able to attack, and won't do so.