Effrontery (5)
'effrontery' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for nerve that I've seen before include "Hold it and maintain courage" , "Coolness under fire" , "Courage, like bodily fibre" , "Bundle of fibres in the body (5)" , "Bodily fibre carrying sensation" .)
(Other definitions for brass neck that I've seen before include "Effrontery" , "Offering cheek" .)