"The final frontier" (5)
I believe the answer is:
space
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the final frontier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for space that I've seen before include "Empty area, may be outer" , "Unfilled region" , "Interstellar region" , ""The final frontier"" , "Interstellar vacuum" .)