Scottish king is able to follow horse (6)
I believe the answer is:
duncan
'scottish king' is the definition.
(I've seen this before)
'able to follow horse' is the wordplay.
'able' becomes 'can'.
'to follow' says to put letters next to each other (some letters go after or follow others).
'horse' becomes 'dun' (I've seen this before).
'can' put after 'dun' is 'DUNCAN'.
'is' is the link.
(Other definitions for duncan that I've seen before include "innovative dancer" , "Isadora ..., pioneer of modern dance" , "Man's name" , "King murdered by ambitious Macbeth" , "Scottish king" .)