Rotten, like a horse prepared to ride, first to fall away (6)
I believe the answer is:
addled
'rotten' is the definition.
(I've seen this before)
'like a horse prepared to ride first to fall away' is the wordplay.
'like a horse prepared to ride' becomes 'saddled'.
'first to fall away' means to remove the first letter.
'saddled' with its initial letter taken off is 'ADDLED'.
(Other definitions for addled that I've seen before include "Confused like rotten egg" , "spoiled" , "became rotten" , "Confused (wits); made rotten" , "turned bad" .)