Well, I think "Shylock's clutches" makes him sound like some horrific ultra-ogre, which I'm not sure is the most sensitive reading of the text.
Shylock is pressing Antonio in the court to fulfil his bond, made legally, and which Antonio has agreed to: which states that if Antonio does not repay the loan from Shylock within the agreed time, Shylock can cut a pound of flesh from his body. Antonio does not repay the loan - and Shylock is therefore entitled to his bond.
And the court does not dispute this fact - until, of course, Portia realises that the bond does not say anything about blood. Shylock is allowed to cut a pound of Antonio's flesh, but if he removes a single drop of blood, he has taken more than he is entitled to - and the bond is forfeit. And that's how she manages to prevent Antonio from being cut to pieces: even if she does do it at the very last moment.