Bristle, as part of law nullified (3)
I believe the answer is:
awn
'bristle' is the definition.
(botanical term)
'part of law nullified' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'AWN' is hidden within 'law nullified'.
'as' acts as a link.
(Other definitions for awn that I've seen before include "Bristle in flowering part of some grasses" , "Bristle on, eg, barley" , "Bristle on grass spikelet" , "Beard of cereal" , "Bristle-like appendage on grass" .)