Develop key argument (4)
I believe the answer is:
grow
'develop' is the definition.
(I know that develop can be written as grow)
'key argument' is the wordplay.
'key' becomes 'g' (musical key).
'argument' becomes 'row' (to row is to argue).
'g'+'row'='GROW'
(Other definitions for grow that I've seen before include "Develop; increase" , "Become bigger" , "Expand" , "Swell" , "Increase in size" .)