Fibre that transmits instructions to the brain (5)
I believe the answer is:
nerve
'fibre that transmits instructions to the brain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nerve that I've seen before include "Bodily fibre that communicates sensations" , "Bodily fibre that transmits sensations" , "Valour, pluck" , "Mettle, daring" , "Cool courage" .)