A runaway horse is on the beach (6)
I believe the answer is:
ashore
'on the beach' is the definition.
(I've seen this before)
'a runaway horse' is the wordplay.
'runaway' indicates an anagram.
'horse' anagrammed gives 'shore'.
'a'+'shore'='ASHORE'
'is' acts as a link.
(Other definitions for ashore that I've seen before include "Going on land from sea" , "on the coast" , "Hoarse (anag)" , "Home for the sailor" , "on terra firma" .)