Found this on bigdave44.com:
21a Who heralds the onset of ceremonial competition for Spooner’s material (4,5)
{RICE PAPER} – competition is RACE and a PIPER is someone who heralds the onset of ceremonial (as on Burns night, for example). Take the two words and, following the custom of the Rev. Spooner, swap their initial vowels to get a material used in oriental painting.