— Equation; — accelerator (6)
I believe the answer is:
linear
This clue is a double definition.
'equation' is the first definition.
(linear equation is a kind of equation)
'accelerator' is the second definition.
(linear accelerator is a kind of accelerator)
(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" .)