jono
Collins online thesaurus has the definition as a synonym of the solution in the sense (!) of ‘pointless’. I’ve only just looked that up as I didn’t have a problem with it, but thought I should check in the light of your comment.
I agree about the ‘at’ in 10ac - as an ignorant non-scientist that was my only quibble!
Looking at the correct clue now: it appears (from a Google search) to be used in Cornwall as a term for a type of roof formed of slate using the measuring device you and chrise have referred to.