Childishly silly (7)
I believe the answer is:
puerile
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">childishly silly</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 puerile that I've seen before include "Childish, immature" , "Childish, silly, trivial" , "Infantile" , "Babyish" , "Childishly stupid" .)