Don't include what's upsetting in testimony (4)
I believe the answer is:
omit
'don't include' is the definition.
(I've seen this in another clue)
'upsetting in testimony' is the wordplay.
'upsetting' says the letters should be written in reverse.
'in' indicates a hidden word.
'timo' is hidden within 'testimony'.
'timo' written backwards gives 'OMIT'.
'what's' is the link.
(Other definitions for omit that I've seen before include "Fail to use" , "Fail to mention" , "Leave out or exclude" , "Exclude - neglect" , "Miss out" .)