Robert obtaining millions proves a failure (4)
I believe the answer is:
bomb
'a failure' is the definition.
(bomb is a kind of failure)
'robert obtaining millions' is the wordplay.
'robert' becomes 'bob' (short for Robert).
'obtaining' indicates putting letters inside.
'millions' becomes 'm' (abbreviation).
'bob' going around 'm' is 'BOMB'.
'proves' is the link.
(Other definitions for bomb that I've seen before include "Load of money (slang)" , "Exploding device" , "worn-out car" , "Mine" , "not to be hit!" .)