Top part covering Aintree’s second horse jump (8)
I believe the answer is:
capriole
'horse jump' is the definition.
(I've seen this before)
'top part covering aintree's second' is the wordplay.
'top' becomes 'cap' (cap is a kind of top).
'part' becomes 'role' (eg a theatrical role).
'covering' is an insertion indicator.
'aintree's second' becomes 'i' (2nd letter of 'aintree').
'role' placed around 'i' is 'riole'.
'cap'+'riole'='CAPRIOLE'
(Other definitions for capriole that I've seen before include "Horse's jump" , "Ballet move" , "Horse jump" , "High jump" , "Leap" .)