Fuel cell's casing holds ordinary acid, primarily (4)
I believe the answer is:
coal
'fuel' is the definition.
(I know that coal is a type of fossil fuel)
'cell's casing holds ordinary acid primarily' is the wordplay.
'casing' means to remove the middle letters (the outside letters are the case of the word).
'holds' indicates putting letters inside.
'primarily' suggests taking the first letters.
'cell' with its centre removed is 'cl'.
The first letters of 'ordinary acid' is 'oa'.
'cl' placed around 'oa' is 'COAL'.
(Other definitions for coal that I've seen before include "Fossil fuel" , "This consists mainly of carbon" , "Carbonized vegetable matter" , "slack" , "Char" .)