Second one to leave United got on his horse? (7)
I believe the answer is:
mounted
'got on his horse?' is the definition.
(mounted a horse)
'second one to leave united' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'one' becomes 'I' (Roman numeral).
'to leave' suggests deleting specific letters.
'united' with 'i' taken away is 'unted'.
'mo'+'unted'='MOUNTED'
(Other definitions for mounted that I've seen before include "Riding a horse" , "Prepared for exhibition" , "Scaled" , "On horseback" , "On a steed" .)