Lemon (3)
'lemon' is the definition.
(both can mean a poor example of something)
This is the entire 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 drop that I've seen before include "Small drink" , "Bit of water" , "See 1" , "Sweet; fall" , "Unload" .)