Develop good line (4)
I believe the answer is:
grow
'develop' is the definition.
(I know that develop can be written as grow)
'good line' is the wordplay.
'good' becomes 'g' (abbreviation).
'line' becomes 'row' (a row or line of things).
'g'+'row'='GROW'
(Other definitions for grow that I've seen before include "Mature" , "Develop - propagate" , "Burgeon" , "Wax" , "Get bigger" .)