"Fill in the ___" (6)
(Other definitions for blank that I've seen before include "Vacuous" , "Bare; not decorated" , "Not written on; (look) uncomprehending" , "Void (of writing, thought, etc)" , "Not written or printed on" .)
(Other definitions for blanks that I've seen before include "Cartridges with explosive charge but no bullet" , "Explosive cartridges with no bullets" , "Vacancies" .)