Highway rig (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">highway rig</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 semi that I've seen before include "House joined to another" , "Dwelling" , "place" , "home perhaps" , "Prefix meaning half (4)" .)