Develop gradually (6)
I believe the answer is:
evolve
'develop gradually' is the definition.
(evolution is gradual development)
This is all the clue.
(Other definitions for evolve that I've seen before include "Develop by natural selection" , "Unravel" , "Develop gradually over a long time" , "Change gradually to present condition" , "Develop or change very gradually (6)" .)