After brown bread, nothing is a lost cause (4,4)
I believe the answer is:
dead duck
'a lost cause' is the definition.
(term for a hopeless or unsuccessful thing)
'after brown bread nothing' is the wordplay.
'after' is a charade indicator (letters next to each other).
'brown bread' becomes 'dead' (Cockney rhyming slang).
'nothing' becomes 'duck' (I've seen this before).
'dead' put next to 'duck' is 'DEAD DUCK'.
'is' is the link.
(Other definitions for dead duck that I've seen before include "one certain to fold" , "Useless thing or person" , "It's useless or failed, like moribund mallard" , "Moribund mallard as failure" , "non-starter" .)