A horse at sea and on land (6)
I believe the answer is:
ashore
'on land' is the definition.
('ashore' can be a synonym of 'on land')
'a horse at sea' is the wordplay.
'at sea' indicates an anagram.
'horse' is an anagram of 'shore'.
'a'+'shore'='ASHORE'
'and' is the link.
(Other definitions for ashore that I've seen before include "Hoarse (anag)" , "On(to) dry land" , "Home for the sailor" , "beach" , "Toward land from the sea" .)