Run commercial on two lines (5)
I believe the answer is:
radii
'lines' is the definition.
(radius is a kind of line)
'run commercial on two' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'commercial' becomes 'ad'.
'on' says to put letters next to each other.
'two' becomes 'ii' (roman numeral).
'r'+'ad'+'ii'='RADII'
(Other definitions for radii that I've seen before include "Halves of diameter" , "Half-diameters" , "Lines from a circle's centre to the circumference" , "Lines drawn from a circle's centre" , "Half diameters" .)