(TMTSF)2ClO4 is a quasi-one-dimensional organic conductor and superconductor with Tc=1.4K, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the Se77 NMR Knight shift at low fields reveals a decrease in spin susceptibility χs consistent with singlet spin pairing. The field dependence of the spin-lattice relaxation rate at 100mK exhibits a sharp crossover (or phase transition) at a field Hs∼15kOe, to a regime where χs is close to the normal state value, even though Hc2Hs.