Races through homework, a simple piece (5)
I believe the answer is:
ditty
'a simple piece' is the definition.
'races through homework' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'through' means one lot of letters goes inside another (I've seen this in other clues).
'homework' becomes 'diy' (DIY is work done on your home).
'tt' placed within 'diy' is 'DITTY'.
(Other definitions for ditty that I've seen before include "Little tune" , "Short simple song" , "Air" , "Light-hearted song" , "Simple tune" .)