Gitto - I was more-or-less at that exact point earlier. I found that starting with 7-letter answers that did seem to fit, and working from there, eventually delivered the PDM.
Very neat puzzle. I love the twist that creates the issues with clue lengths. Not much of a PDM for me, alas. I had no idea what was going on until I had filled in nearly all the grid and begun to derive the instruction.
Just got round to looking at this and was put off by the unclued element in a jigsaw type puzzle. I came here to gauge the consensus prior to printing out. Glad I did. For this solver it goes into the "can't be a***d" category. Plenty of other puzzly fish in the sea!
Are any of the final entries real words, otherwise I have a monumental problem. I'm guessing that it is the same four letters, but why? I cannot work out.