Since both chlorine and fluorine belong to the halogen group (they are group 17 elements), both their valence shells contain 7 electrons. Since the difference in the electronegativities of chlorine and fluorine is not extremely high, the chemical bonds formed between chlorine and fluorine atoms are highly covalent in nature. In a ClF5 molecule, chlorine is the central atom featuring five single bonds with fluorine atoms. Since chlorine has 7 electrons in the valence shell, the remaining two electrons form a lone pair.
As per the VSEPR theory (valence shell electron pair repulsion theory), the shape of a ClF5 molecule is square pyramidal.