  We propose a simple method for generating spin squeezing of atomic ensembles
in a Floquet cavity subject to a weak, detuned two-photon driving. We
demonstrate that {\it the weak squeezing of light inside the cavity can,
counterintuitively, induce strong spin squeezing}. This is achieved by
exploiting the anti-Stokes scattering process of a photon pair interacting with
an atom. Specifically, {\it one photon of the photon pair is scattered into the
cavity resonance by absorbing partially the energy of the other photon whose
remaining energy excites the atom}. The scattering, combined with a Floquet
sideband, provides an alternative mechanism to implement Heisenberg-limited
spin squeezing. Our proposal does {\it not} need multiple classical and
cavity-photon drivings applied to atoms in ensembles, and therefore its
experimental feasibility is greatly improved compared to other cavity-based
schemes. As an example, we demonstrate a possible implementation with a
superconducting resonator coupled to a nitrogen-vacancy electronic-spin
ensemble.
