Oracle holding information back breaks a promise (7)
I believe the answer is:
reneges
'breaks a promise' is the definition.
(I've seen this in another clue)
'oracle holding information back' is the wordplay.
'oracle' becomes 'seer' (I've seen this before).
'holding' means one lot of letters goes inside another.
'information' becomes 'gen' (gen can informally mean information).
'back' is a reversal indicator.
'seer' going around 'gen' is 'segener'.
'segener' in reverse letter order is 'RENEGES'.
(Other definitions for reneges that I've seen before include "Fails to fulfil a promise or obligation" , "Goes back" .)