Fight verbally
(Other definitions for argue that I've seen before include "Make a case" , "Heatedly exchange views" , "Debate or disagree" , "Contend with reasoning" , "Bandy words" .)
'fight verbally' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for spar that I've seen before include "Crystalline mineral" , "Rafter, boom" , "Sticks" , "Box (lightly) - fight (verbally)" , "Shadow box" .)