Certain tray's contents

ASH

What remains after burning might be found in this shallow container.

This clue plays on the word "tray" to hint at an ashtray, which is a specific type of tray. The "contents" of an ashtray are typically ash, so the answer is simply what you’d find inside. It’s a straightforward example of a container clue, where the first word ("certain") just signals you’re looking for a specific kind of tray. The wordplay relies on the common association between trays and ashtrays in everyday language.