Expand key argument (4)
I believe the answer is:
grow
'expand' is the definition.
(I've seen this before)
'key argument' is the wordplay.
'key' becomes 'g' (I've seen this in other clues**).
'argument' becomes 'row' (I've seen this before).
'g'+'row'='GROW'
(Other definitions for grow that I've seen before include "Develop; increase" , "Become larger" , "Wax" , "Swell" , "Increase in size" .)