Grow quickly (8)
I believe the answer is:
mushroom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grow quickly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mushroom that I've seen before include "See 9" , "Proliferate" , "food in the field" , "Grow rapidly" , "develop rapidly" .)