Possible Clues:

• Chew over what the chippy has for starters? It has a number of pieces and it comes in a box!
• Challenge commercialized by mapmaker John Spilsbury circa 1760
• Fitting activity?
• Fitting pastime perhaps for those in 1 across and 19 down
• Picture cut into interlocking pieces for reassembly
• Sounds like carpenter problem needs to be assembled at leisure
• Pieceful pastime?
• What the theme answers' 'extra pieces' spell
• Fitting pastime for one of those cutting corners to chew over
• Amusement that involves interlocking pieces