Abnormally high body temperature (5)
I believe the answer is:
fever
'abnormally high body temperature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fever that I've seen before include "Intense nervous excitement or anticipation?" , "State of heightened excitement" , "Pyrexia" , "High-temperature illness" , "Nervous excitement, high temperature" .)