Multitudes some author described (6)
I believe the answer is:
hordes
'multitudes' is the definition.
(I know this)
'some author described' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'HORDES' is hidden in the letters of 'author described'.
(Other definitions for hordes that I've seen before include "Horsed around with the multitudes" , "Vast multitudes" , "Huge noisy crowds" , "Many" , "Mobs of people in Rhodes" .)