Greatly (4)
I believe the answer is:
very
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greatly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for very that I've seen before include "To a large extent" , "Real" , "Frightfully" , "Truly" , "jolly British!" .)