Forget moment climbing over it (4)
I believe the answer is:
omit
'forget' is the definition.
(to omit to do something is to forget to do it)
'moment climbing over it' is the wordplay.
'moment' becomes 'mo' (as in 'just a mo').
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'mo' in reverse letter order is 'om'.
'om'+'it'='OMIT'
(Other definitions for omit that I've seen before include "Disregard" , "Leave out or overlook" , "Drop" , "Exclude; veto" , "Leave something out" .)