A run around part of the Circle Line? (3)
I believe the answer is:
arc
'the circle line?' is the definition.
(an arc is a line forming part of a circle)
'a run around' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'around' becomes 'c' (abbreviation for 'circa').
'a'+'r'+'c'='ARC'
'part of' is the link.
(Other definitions for arc that I've seen before include "Electrical spark" , "Rainbow, e.g." , "less than the complete circuit" , "Curved section" , "Part of circle; discharge" .)