Clue: School's top people finally cracking Nutmeg's riddle (5)
Answer: SIEVE
Explanation: Definition: riddle
A charade of S ('**S**chool's') plus IEVE, an envelope ('cracking') of E ('peopl**E** finally') in I'VE ('Nutmeg's' i.e. Nutmeg has). Here I had a nudge from 3D in Bluth's recent Independant crossword 11,283.