It’s dead skinny! (7)
I believe the answer is:
cuticle
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for cuticle that I've seen before include "finger has one" , "Dead skin on hand" , "Dead skin at the base of a fingernail" , "Dead skin at nail's base" , "Skin around the nail" .)