The energy of the first excited state in the neutron-rich N=28 nucleus Cl45 has been established via in-beam γ-ray spectroscopy following proton removal. This energy value completes the systematics of the E(1/21+)-E(3/21+) level spacing in odd-mass K, Cl, and P isotopes for N=20-28. The results are discussed in the framework of shell-model calculations in the sd-fp model space. The contribution of the central, spin-orbit, and tensor components is discussed from a calculation based on a proton single-hole spectrum from G-matrix and π+ρ meson exchange potentials. A composite model for the proton 0d3/2-1s1/2 single-particle energy shift is presented.