An upright Leicester supporter (7)
I believe the answer is:
apostle
'supporter' is the definition.
(apostle is a kind of supporter)
'an upright leicester' is the wordplay.
'an' becomes 'a'.
'upright' becomes 'post' (upright can mean a vertical post).
'leicester' becomes 'le' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'post'+'le'='APOSTLE'
Can you help me to learn more?
(Other definitions for apostle that I've seen before include "Vigorous campaigner" , "Maybe Peter" , "Preacher of the gospel" , "A follower of Jesus" , "E.g. St Paul" .)