Pastille, Araucaria (John Graham) is the Guardian's longest-serving setter. Humorous is right - but he can be maddening. This puzzle is one of his more fiendish offerings.
A bit of background:
http://www.guardian.co.uk/g2/story/0,3604,438558,00.html
and
http://en.wikipedia.org/wiki/John_Galbraith_Graham
Don't expect him to follow conventional rules. His style is libertarian, even anarchic, rather than what's often called 'Ximenean'. But he's also quite rigorous and meticulous.
Here, the asterisked clues (which lack further definition) lead to a keyword. Not to beat about the bush, because it's been made public on AnswerBank if not on here, that keyword is 'Potter' and it refers to literary characters (authors and their work) and to those who mess around with clay.
For example. a clue of this type is 11a (grid A): Not talking to the gilded French. The answer's DUMB + LE + DORE = a character from a well-known work associated with the theme word.
The more 'straightforward' clues rely on wordplays which are often rather wacky, but you'll find some familiar devices - anagrams, double definitions, homophones, hidden clues etc - among them.
I hope you enjoy this and other Araucaria/Guardian offerings along with your well-earned rest from the allotment.