Horse runs around in tar (7)
I believe the answer is:
mariner
'tar' is the definition.
(tar can mean a sailor)
'horse runs around in' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'runs' becomes 'r' (cricket abbreviation).
'around' indicates putting letters inside.
'mare'+'r'='marer'
'marer' placed around 'in' is 'MARINER'.
(Other definitions for mariner that I've seen before include "Seaman" , "'''The Rime of the Ancient ...'' (Coleridge) (7)'" , "Sea dog" , "See 13" , "Probe" .)