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' means one lot of letters goes inside another.
'millions' becomes 'm' (abbreviation for millions).
'bob' enclosing 'm' is 'BOMB'.
'proves' acts as a 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!" .)