Develop gradually (6)
I believe the answer is:
evolve
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">develop gradually</span>' is the definition.<br/ ><span class="explanation-format__explanation">(evolution is gradual development)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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)" .)