Leave out, fail to do (4)
I believe the answer is:
omit
I believe this is a double definition.
'leave out' is the first definition.
(I know that leave out can be written as omit)
'fail to do' is the second definition.
(I've seen this in another clue)
(Other definitions for omit that I've seen before include "Leave out or fail to do" , "Skip" , "Leave out or neglect" , "by-pass" , "Fail to include" .)