Expand key argument (4)
I believe the answer is:
grow
'expand' is the definition.
(to grow is to expand in size)
'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 larger" , "Wax" , "Swell" , "Increase in size" .)