A small, young horse left shed for racecourse (5)
I believe the answer is:
ascot
'racecourse' is the definition.
(I've seen this before)
'a small young horse left shed' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'young horse' becomes 'colt' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'shed' suggests deleting specific letters ('shed' as in 'cut away').
'colt' with 'l' taken away is 'cot'.
'a'+'s'+'cot'='ASCOT'
'for' acts as a link.
(Other definitions for ascot that I've seen before include "Berkshire town, famous for horse racing" , "prestigious meeting" , "English horse racing venue on coast" , "Broad necktie" , "Famous racecourse" .)