Goes back on purpose, generally backing up just a bit (7)
I believe the answer is:
reneges
'goes back on' is the definition.
('renege' can be a synonym of 'go back on')
'purpose generally backing up just a bit' is the wordplay.
'just a bit' says the answer is hidden in the clue.
'backing up' says the letters should be written backwards.
'segener' is hidden within 'purpose generally'.
'segener' written backwards gives 'RENEGES'.
(Other definitions for reneges that I've seen before include "Fails to fulfil a promise or obligation" , "Goes back" .)