compound
4-vinyl guaiacol
The phenol responsible for clove character, produced by yeast strains carrying an active POF gene.
Formed when yeast decarboxylates ferulic acid released from malt. Most brewing strains lack the gene and are described as POF-negative; Weissbier, witbier and Belgian strains have it.
A ferulic acid rest at around 44 °C in the mash releases more of the precursor, which is how brewers dial clove character up deliberately.
Evidence
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The clove-like character of wheat beers and saisons comes from 4-vinyl guaiacol and related compounds, produced by yeast strains carrying a functional phenolic-off-flavour gene — which most clean ale and lager strains have lost or had inactivated during domestication.
Backed by a source
Sources: Yeast: The Practical Guide to Beer Fermentation — print; Brewing: Science and Practice — print