Neglect makes Timothy much upset! (4)
I believe the answer is:
omit
'neglect' is the definition.
(I know this)
'timothy much upset' is the wordplay.
'timothy' becomes 'tim' (I've seen this in other clues).
'much' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'upset' says the letters should be written backwards.
'tim'+'o'='timo'
'timo' in reverse letter order is 'OMIT'.
'makes' is the link.
Can you help me to learn more?
(Other definitions for omit that I've seen before include "Neglect to include" , "Exclude; forget to include" , "Leave out, perhaps in error" , "Leave out or forget" , "Disregard" .)