Exhausted American editor on horseback (4,2)
I believe the answer is:
'exhausted' is the definition.
('used-up' can be similar in meaning to 'exhausted')
'american editor on horseback' is the wordplay.
'american' becomes 'us'.
'editor' becomes 'ed'.
'on horseback' becomes 'up'.
(Other definitions for used up that I've seen before include "Consumed, finished" , "Exhausted the supply" , "Consumed completely" , "Spent" .)