Underground bar making a comeback, ending in failure (4)
I believe the answer is:
tube
'underground' is the definition.
(the London Underground is nicknamed the Tube)
'bar making a comeback ending in failure' is the wordplay.
'bar' becomes 'but' (both can mean 'except for').
'making a comeback' shows that the letters should be reversed in order.
'ending in' suggests the final letters.
The final letter of 'failure' is 'e'.
'but' backwards is 'tub'.
'tub'+'e'='TUBE'
(Other definitions for tube that I've seen before include "TV" , "Pipe - railway" , "appearing tinny?" , "Underground transport" , "Valve" .)