@Paul and @Quisling,
Well… as I said I sent a note to the author Jasper Fforde and he was kind enough to respond to me just now. Here’s exactly what he said which I will copy and paste…
“Hi Jono,
Yes it does have an answer, and yes it is a cryptic clue.
It's not the first time I've used crossword clues but left the answer blank. Don't know why. Despite being a writer, I am terrible at crosswords.
In 'Shades of Grey' there is this one:
“Three down,” he muttered, “I spy an Equus. Nine letters, begins and ends in A.”
...which should be obvious, and might help you solve the plumber clue. They *are* cryptic, but best not overthink it, and there's probably a silly joke or pun involved.
The actual answer? I wish I could remember. I may, given a bit of time. If I come up with it, I'll let you know.
Or offer a clue
hmmm...
Jasper”