Right, so roll? at the end is your definition, question mark and all - a burger bun being that type of bread roll. For the wordplay, grab the letters of "gum rubber", chuck out the M (that's "wanting metre" telling you to drop M), then bung in an N for "new". Then "Supply" is doing the anagram work here - think supplely, i.e. flexibly - shuffle it all up and you land on BURGER BUN.