Forget Im about to reverse (4)
I believe the answer is:
omit
'forget' is the definition.
(to omit to do something is to forget to do it)
'im about to reverse' is the wordplay.
'about' is an insertion indicator.
'reverse' shows that the letters should be reversed in order.
'im' going into 'to' is 'timo'.
'timo' backwards is 'OMIT'.
(Other definitions for omit that I've seen before include "Fail to mention" , "Leave out or fail to do" , "Leave out or neglect" , "by-pass" , "Exclude; veto" .)