Horse running by the sea (5)
I believe the answer is:
shore
'by the sea' is the definition.
'horse running' is the wordplay.
'running' is an anagram indicator (the letters run around to different places).
'horse' anagrammed gives 'SHORE'.
(Other definitions for shore that I've seen before include "Country" , "Horse (anag.)" , "Seafront" , "Land by water" , "Land along the edge of sea or lake" .)