Enlarge upon (6)
I believe the answer is:
expand
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">enlarge upon</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for expand that I've seen before include "Distend" , "balloon" , "Broaden" , "produce growth" , "Enlarge - stretch - become more friendly" .)