Translation

ぎゅっと握る / 食いしばる / 固く締める

Meaning

Clench means to close or tighten something strongly, especially your fist, teeth, jaw, or muscles. It often shows anger, fear, pain, or effort.

Examples

• 彼は怒りで拳をぎゅっと握りました。
  He clenched his fists in anger.
• 彼女は痛みに耐えながら歯を食いしばりました。
  She clenched her teeth against the pain.