Half the alphabet? This was split some time ago (4)
I believe the answer is:
atom
This clue is a double definition.
'half the alphabet?' is the first definition.
(A to M, the first 13 letters of the alphabet)
'this was split some time ago' is the second definition.
(as in splitting the atom)
(Other definitions for atom that I've seen before include "A tiny particle but a big bomb" , "Basic unit of a chemical element" , "Small amount of matter" , "Small particle of matter" , "Tiny particle, great bomb" .)