Stranger dismissed - rides off (8)
I believe the answer is:
outsider
'stranger' is the definition.
(I know that outsider is a type of stranger)
'dismissed rides off' is the wordplay.
'dismissed' becomes 'out' (dismissed in cricket).
'off' indicates an anagram.
'rides' with letters rearranged gives 'sider'.
'out'+'sider'='OUTSIDER'
(Other definitions for outsider that I've seen before include "One who doesn't belong (or isn't fancied)" , "One not part of group" , "Not one of the favourites" , "Improbable winner" , "expected to be unsuccessful" .)