Highly productive (6)
I believe the answer is:
fecund
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">highly productive</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 fecund that I've seen before include "Highly fertile" , "Prolific, fertile" , "Highly productive" , "Fertile or productive" , "Producing many offspring or ideas" .)