Mint producing something less than the King's Shilling? (10)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

there were I suppose twelve pennies in a King’s shilling, which is  a slang term referring to the advance (earnest) payment, which used to be given to recruits in the UK armed forces; PENNYROYAL is a species of flowering plant in the mint family. I assumed it was something you sucked…