Cultivate grand argument (4)
I believe the answer is:
grow
'cultivate' is the definition.
(growing is a kind of cultivating)
'grand argument' is the wordplay.
'grand' becomes 'g' (synonyms).
'argument' becomes 'row' (I've seen this before).
'g'+'row'='GROW'
(Other definitions for grow that I've seen before include "Get bigger" , "Increase in size" , "Expand" , "Wax" , "Become larger" .)