Line of argument? (3)
I believe the answer is:
row
This clue is a double definition.
'line' is the first definition.
(a row or line of things)
'argument?' is the second definition.
(to row is to argue)
'of' is the link.
(Other definitions for row that I've seen before include "Quarrel about the tier" , "Use an oar" , "Disagreement" , "Line, tier" , "Altercation" .)