The phrase must be "April fool", surely. However I seem to have lost the first "l". Where does that feature in the clues/grid? I'm also unsure of 21a ("after removal of limits Europe modified former register"). This has to be "roup": (E)urop(e) - anag, but how does the definition fit this?
Also 22a: "Low starting point - it makes bar give attention". This seems to be "base", but how does the second part of the clue fit this?