fear, be afraid of, dread
惧 centers on a deep emotional reaction to something threatening: fear, dread, or being afraid of something.
惧 combines 忄 (heart/mind) with 具, which likely contributes the sound and may relate to the idea of being fully affected by fear.
The heart radical 忄 beside 具 suggests a heart fully occupied by fear: when you are afraid, your heart is filled with dread.
For ク, imagine a cool, creeping dread that makes you shiver: cool -> ク, and the fear fills your heart.