Normal, presumably, to inject sulphur to bring pet relief (7)
I believe the answer is:
nostrum
'relief' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nostrum that I've seen before include "Phoney panacea" , "Favourite remedy or scheme" , "secret potion" , "Some think it helps" , "Quack cure" .)