Developed gradually (7)
I believe the answer is:
evolved
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">developed gradually</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for evolved that I've seen before include "Gradually developed as Darwin put it" , "Underwent development" , "Developed gradually over time" , "Undergone stages of development" , "Grew very gradually as Darwin hypothesised" .)