One opposing some Italian timekeeping (4)
I believe the answer is:
anti
'one opposing' is the definition.
(prefix meaning 'opposing')
'some italian timekeeping' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ANTI' is hidden in the letters of 'italian timekeeping'.
(Other definitions for anti that I've seen before include "Person opposed" , "not favoured" , "(Person who is) not in favour" , "Against, informally" , "Prefix meaning against" .)