Lemon (3)
'lemon' is the definition.
(both can mean a poor example of something)
This is all the clue.
(Other definitions for dud that I've seen before include "Something useless - a palindrome" , "A thing failing to work" , "Useless, doesn't work - a palindrome" , "Bullet that fails to fire" , "Not working" .)
(Other definitions for law that I've seen before include "Set of rules laid down by authority" , "Collection of rules imposed by authority" , "Edict" , "Official rule" , "Commandment" .)
(Other definitions for drop that I've seen before include "Fall down after prod" , "Unload" , "Plummet" , "Not a lot of water" , "Spot" .)