Dark horse seen over eastern hill at coast? (4)
I believe the answer is:
dune
'hill at coast?' is the definition.
(as in a sand dune on a beach)
'dark horse seen over eastern' is the wordplay.
'dark horse' becomes 'dun' (horse with darkish coat).
'seen over' means one lot of letters go next to another.
'eastern' becomes 'e'.
'dun'+'e'='DUNE'
(Other definitions for dune that I've seen before include "Mound of drifted sand" , "Seashore sand hill" , "Coastal sand hill" , "Nude in the sand" , "Low hill of sand, called in a desert a barkhan" .)