Go back on unrealistic promise we'd made ultimately (6)
I believe the answer is:
recede
'go back' is the definition.
(I've seen this before)
'on unrealistic promise we'd made ultimately' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'ultimately' says to take the final letters.
The final letters of 'unrealistic promise wed made' are 'cede'.
're'+'cede'='RECEDE'
(Other definitions for recede that I've seen before include "Start balding" , "Move backward" , "Decline" , "Decree (anag) - ebb" , "Go back, ebb" .)