Horse, near Aintree's front three, fell maybe (8)
I believe the answer is:
mountain
'fell maybe' is the definition.
(fell can mean a hill or mountain)
'horse near aintree's front' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'near' means one lot of letters go next to another.
'aintree's front' becomes 'ain' (I can't justify this - if you can you should believe this answer much more).
'mount'+'ain'='MOUNTAIN'
'three' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for mountain that I've seen before include "Large surplus stock (eg, of butter)" , "Alp" , "Eminence" , "high position" , "may be in range?" .)