How can an ionic bond have partial covalent in character?
One way is Polarization. When a small cation approaches a large  anion it distorts the charge density towards itself thereby forming a type of bond which has directional characteristic i.e from ** (-) ----> (+) ** which is characteristic of a covalent bond.

This polarization has some rules given by **Fajan ** 


Smaller the cation better distortion/polarization.
More the amount of positive charge more is the polarization.
or more the charge : radius ratio of cation more is the polarization.


So in short every cation has the power of polarization and therefore no bond is 100% ionic.