50% of horses and riders are beastly (6)
I believe the answer is:
horrid
'beastly' is the definition.
(thesaurus)
'50% of horses and riders' is the wordplay.
'50% of' means to take half the letters.
'and' means one lot of letters go next to another.
'riders' becomes 'rid' (I can't justify this - if you can you should give a lot more credence to this answer).
'horses' cut in half is 'hor'.
'hor'+'rid'='HORRID'
'are' acts as a link.
Can you help me to learn more?
(Other definitions for horrid that I've seen before include "Repulsive" , "Appalling" , "Nasty, frightening" , "Repellent" , "Dreadful" .)