German argument to increase (4)
I believe the answer is:
grow
'increase' is the definition.
(I know that grow is a more specific form of the action develop)
'german argument' is the wordplay.
'german' becomes 'g' (abbreviation).
'argument' becomes 'row' (to row is to argue).
'g'+'row'='GROW'
'to' acts as a link.
(Other definitions for grow that I've seen before include "Mature" , "Become larger" , "Multiply" , "Burgeon" , "Become bigger" .)