This is a clever &lit-style clue where the definition is energy briefly produced from this — oil shale is indeed a substance from which energy is produced. For the wordplay, "briefly" tells us to shorten "energy" to just E, giving us the fodder "Hallo is e". The word "produced" acts as the anagram indicator, and rearranging the letters of HALLOISE gives us OIL SHALE. Neat surface reading too — it sounds like someone asking a question!