I have struggled to the finishing line. Thanks for all the hints.
Drxx describes the puzzle as devious. I’m not sure if this is a pejorative word to apply to a cryptic crossword! Unfortunately I think it is far worse than that.
To avoid extra letters in the four-word phrase, the line coming from the NW corner of the grid will have to pass precisely through the NW corner of the blank cell. So it will not meet the other line at the centre of the blank cell! The dot must therefore be placed nearer to the NW corner of the blank cell.
Is this devious? I would say unfair, or perhaps inappropriate for a puzzle like this.