Horse able to be Shakespearean character (6)
I believe the answer is:
duncan
'shakespearean character' is the definition.
(King Duncan from Macbeth)
'horse able' is the wordplay.
'horse' becomes 'dun' (I've seen this before).
'able' becomes 'can'.
'dun'+'can'='DUNCAN'
'to be' is the link.
(Other definitions for duncan that I've seen before include "innovative dancer" , "Man's name" , "Isadora ..., pioneer of modern dance" , "Scottish king" , "King murdered by ambitious Macbeth" .)