Have finally twigged what's going on here after far too long - but am still struggling with parsing and answer for18ac - the middle letter in fact. I can see one that fits with 7 but not sure at all. Can anyone provide discreet enlightenment?
Also with 2d, I assume the definition occurs later rather than earlier in the clue?
Turned out to be fun in the end - and cleverly done. Took me ages to decipher the message.