Just plain silly (5)
I believe the answer is:
inane
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">just plain silly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for inane that I've seen before include "Lacking meaning" , "Silly, witless" , "Devoid of intelligence" , "Foolish, silly" , "Stupid and silly" .)