I blundered, needing ultimately to go up backwards (4)
I believe the answer is:
oops
'i blundered' is the definition.
('oops' is said to acknowledge a mistake)
'ultimately to go up backwards' is the wordplay.
'ultimately' says to take the final letters.
The last letters of 'to go up backwards' are 'OOPS'.
'needing' is the link.
(Other definitions for oops that I've seen before include "Oh dear, I've spilt something!" , "statement after accident" , "Exclamation on making obvious mistake" , "Exclamation on making a minor mistake" , "Oh dear, that was careless!" .)