With what neutral molecule is ClO^– isoelectronic? Is that molecule a Lewis base?

ClO^- isisoelectronic to ClF as both the compounds contain 26 electrons in all.

ClO^- : 17+8+1 = 26

ClF : 17+9 = 26

Yes, ClF Molecule is a Lewis base as it accepts electrons from F to form ClF₃.