Strange, frightening (5)
I believe the answer is:
eerie
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">strange</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(eerie means odd or strange)</span><br/ ><br/ >'<span class="explanation-format__original">frightening</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for eerie that I've seen before include "Ghostly" , "Spooky, creepy" , "Uncannily strange" , "Sinister" , "Mysterious" .)