Advertising something knights wore on horse (4,4)
I believe the answer is:
junk mail
'advertising something knights' is the definition.
I can't tell whether this definition defines the answer.
'wore on horse' is the wordplay.
'wore' becomes 'mail' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'horse' becomes 'junk' (both can mean heroin).
'mail' put after 'junk' is 'JUNK MAIL'.
Can you help me to learn more?
(Other definitions for junk mail that I've seen before include "post to ditch" , "Unwanted letters" , "Unsolicited letters" , "Unsolicited stuff" , "Unsolicited post, such as advertising etc" .)