Roll up, going, at first, round about part of a mile (7)
I believe the answer is:
furlong
'part of a mile' is the definition.
('furlong' is part of 'mile')
'roll up going at first round' is the wordplay.
'roll up' becomes 'furl' ('furl' can be a synonym of 'roll up').
'going at' becomes 'ng' (I can't justify this - if you can you should give a lot more credence to this answer).
'first' is a charade indicator (letters next to each other).
'round' becomes 'o' (round can mean a round object).
'ng' put after 'o' is 'ong'.
'furl'+'ong'='FURLONG'
'about' is the link.
Can you help me to learn more?
(Other definitions for furlong that I've seen before include "Imperial unit of length, an eighth of a mile" , "Measure of length" , "1/8th of a mile" , "One eighth of a mile" , "Measure of distance" .)