— Equation; — accelerator (6)
I believe the answer is:
linear
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">equation</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(linear equation is a kind of equation)</span><br/ ><br/ >'<span class="explanation-format__original">accelerator</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(linear accelerator is a kind of accelerator)</span><br/ ><br/ ></span>
(Other definitions for linear that I've seen before include "Nailer (anag.)" , "Consisting of lines" , "In one dimension, I learn (anag.)" , "Long and narrow, of one dimension" , "Straight; sequential" .)