Temerity (5)
I believe the answer is:
nerve
'temerity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nerve that I've seen before include "Body fibre conveying sensations to brain" , "Coolness under fire" , "Sauce" , "Courage - cheek" , "Courage, like bodily fibre" .)