Resistance units (4)
I believe the answer is:
ohms
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">resistance units</span>' is the definition.<br/ ><span class="explanation-format__explanation">(ohm is a kind of resistance unit)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ohms that I've seen before include "Frank on official post" , "Electricity" , "measures of resistance" , "Frank marking" , "Letters with official status in correspondence" .)