Some from Ithaca to exclude (4)
I believe the answer is:
'exclude' is the definition.
(I know that exclude can be written as omit)
'some from ithaca to' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'OMIT' is hidden within 'from ithaca to'.
(Other definitions for omit that I've seen before include "Leave out or exclude" , "Leave out, do not include (4)" , "Fail to include" , "Neglect to include" , "Leave out or undone" .)