School's top people finally cracking Nutmeg's riddle (5)
I believe the answer is:
sieve
'riddle' is the definition.
(I know that riddle has the is a more specific form of the action sieve)
'school's top people finally cracking nutmeg's' is the wordplay.
'top' says to take the initial letters (in a down clue, the first letter is top).
'finally' says to take the final letters.
'cracking' is an insertion indicator (inserted letters go into a gap or crack).
'nutmeg's' becomes 'ive' (where the crossword setter is 'Nutmeg').
The first letter of 'schools' is 's'.
The last letter of 'people' is 'e'.
'e' placed inside 'ive' is 'ieve'.
's'+'ieve'='SIEVE'
(Other definitions for sieve that I've seen before include "Riddle, strain" , "Straining device" , "Mesh used for straining or sifting" , "Utensil for straining or sifting food" , "Jumblies' vessel" .)