Agent called before, one day turning up earlier (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A charade of NOMI, a reversal (‘turning up’ in a down light) of I MON (‘one day’ – Monday) plus NÉE (‘called before’). One definition of a NOMINEE is a person who acts on behalf of another, especially to keep the latter’s identity hidden.