One sending brainy message in bottle? (5)
I believe the answer is:
nerve
'bottle?' is the definition.
(both can mean courage)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for nerve that I've seen before include "Strand of body fibre" , "Impulse transmitter - bottle" , "Impudence (colloq.)" , "Boldness" , "Body fibre conveying sensations to brain" .)