An abnormally low body temperature (11)
I believe the answer is:
hypothermia
'an abnormally low body temperature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hypothermia that I've seen before include "Deadly cold" , "dangerous condition" , "Condition of abnormally low body temperature" , "Body condition caused by extreme cold" , "making the blood run cold!" .)