Upsetting some children of individual (3)
I believe the answer is:
one
'individual' is the definition.
'upsetting some children of' is the wordplay.
'upsetting' says the letters should be written in reverse.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'eno' is hidden in the letters of 'children of'.
'eno' backwards is 'ONE'.
(Other definitions for one that I've seen before include "SECOND PART OF 16" , "I can mean this" , "Single, sole" , "Individual" , "doesn't count for much" .)