Top horse failing to finish in races (1-5)
I believe the answer is:
t-shirt
'top' is the definition.
(I know that T-shirt is a type of shirt)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for t-shirt that I've seen before include "Informal collarless top" , "Casual upper garment" , "Short-sleeved cotton top with no collar" , "Casual vest-like garment much worn by youth" , "Light short-sleeved top" .)