Possible Clues:

• What's-his-name; a nuisance
• 'Such-and-such,' but for names
• That confounded person may be one of 20 down
• What the wretched 2 down keeps doing, by the sound of it
• The whatsit for the SOS?
• An SOS form whatshisname?
• Person unspecified or abjured
• Real blankety-blank
• You've forgotten the name of 20 down, perhaps
• Unpleasant person - whose name has been forgotten!