Line of argument? (3)
I believe the answer is:
row
I believe this is a double definition.
'line' is the first definition.
(I've seen this before)
'argument?' is the second definition.
(thesaurus)
'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" .)