Skip round little boy, jumping up (4)
I believe the answer is:
omit
'skip' is the definition.
(I've seen this before)
'round little boy jumping up' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'little boy' becomes 'tim'.
'jumping up' says the letters should be written in reverse (in a down clue, letters go up).
'tim' backwards is 'mit'.
'o'+'mit'='OMIT'
(Other definitions for omit that I've seen before include "Leave out or exclude" , "Leave out or fail to do" , "Leave out or neglect" , "Skip" , "by-pass" .)