Horse that's old and imperfect (7)
I believe the answer is:
damaged
'imperfect' is the definition.
(associated in meaning)
'horse that's old' is the wordplay.
'horse' becomes 'dam' (I can't explain this - if you can you should believe this answer much more).
'that's old' becomes 'aged' (similar in meaning).
'dam'+'aged'='DAMAGED'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for damaged that I've seen before include "Flawed, harmed" , "Hurt" , "Impaired" , "Sullied or harmed" , "in poor condition" .)