Lawman and horse, that is (7)
I believe the answer is:
mountie
'lawman' is the definition.
(I know that Mountie is a type of officer)
'horse that is' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'mount'+'ie'='MOUNTIE'
'and' acts as a link.
(Other definitions for mountie that I've seen before include "One boasting of getting man" , "The lawman always gets his man!" , "Horse-riding Canadian cop" , "North American policeman" , "Canadian policemen" .)