Develop grand argument (4)
I believe the answer is:
grow
'develop' is the definition.
(growing is a kind of developing)
'grand argument' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'argument' becomes 'row' (I've seen this before).
'g'+'row'='GROW'
(Other definitions for grow that I've seen before include "Get bigger" , "Expand" , "Become bigger" , "Burgeon" , "Wax" .)