More difficult (6)
I believe the answer is:
harder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">more difficult</span>' is the definition.<br/ ><span class="explanation-format__explanation">('hard' is associated in meaning with 'difficult')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for harder that I've seen before include "More tough or difficult" , "More solid" .)