Well al - yes I do, to gull someone is to dupe them, but being a guid Scots word 'loon' doesn't get a mention in Chambers so I can't quite make the connection ! I'm still swithering over 15d with k as the last letter.
Hmm, Mopsy, still not sure.
But 15d., think 5-line "poems", think Edward Lear. Think...
There was a young solver named Mopsy,
Who'd got all the clues at the top, sae
At the end o' the day,
A word ending in "k"
Risked making the solution a flop(sy).