Jelly in a vise is a proverb mentioned in "Asian Figure" by W.S Merwin. What does "Jelly" mean in this proverb? Does it mean gelatinous? Or the jelly that we eat? What kind of clamp is a "Vise"?
It is a metaphor. A Jelly cannot remain intact when tightened in a vise. A vise is like a clamp used to hold things firmly.
Vise - a tool with two parts that can be moved together by tightening a screw so that an object can be held firmly between them while it is being worked on
So to describe a very tight / tense situation this phrase can be used where the subject is in tight scrutiny / focus and cannot hold the pressure mounting on it.
Example usage - "I literally felt like Jelly in a vise to have written the final exams without preparation"