noun

a weakness, especially in a person’s character; a shortcoming.

pride is a terrible failing

fault,shortcoming,weakness,weak point,weak spot,imperfection,defect,flaw,blemish,frailty,infirmity,foible,quirk,idiosyncrasy,vice,deficiency,inadequacy,limitation

preposition

in the absence of; if not.

she longed to be with him and, failing that, to be alone

in the absence of,in default of,lacking,wanting,notwithstanding