P has the higher electronegativity. The bond would be polar covalent with the negative end pointing from Cs to P.
In the Periodic Table, electronegativity increases from left to right and from bottom to top. Thus, the least electronegative elements are at the lower left. The most electronegative elements are at the upper right.
P at the upper right is more electronegative than Cs at the lower left. The Cs bond is polar covalent. The negative end of the Cs→P dipole points from Cs (δ⁺) to P (δ⁻).