Highly rated Eastern horse Nutmeg's invested in (8)
I believe the answer is:
esteemed
'highly rated' is the definition.
'eastern horse nutmeg's invested in' is the wordplay.
'eastern' becomes 'e'.
'horse' becomes 'steed' (steed is a kind of horse).
'nutmeg's invested' becomes 'em' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'steed' placed around 'em' is 'steemed'.
'e'+'steemed'='ESTEEMED'
(Other definitions for esteemed that I've seen before include "Held in high regard, as customer may be" , "Dear" , "attracting veneration" , "Highly admired" , "Honoured" .)