Upright type of horse that covers miles (5)
I believe the answer is:
roman
'upright type' is the definition.
(I've seen this before)
'horse that covers miles' is the wordplay.
'horse' becomes 'roan' (roan is a kind of horse**).
'that covers' is an insertion indicator.
'miles' becomes 'm' (common abbreviation).
'roan' going around 'm' is 'ROMAN'.
'of' is the link.
(Other definitions for roman that I've seen before include "A novel" , "Native to Italian capital" , "Type of candle, nose etc" , "French story" , "Caesar among others" .)