Possible Clues:

• Official - conventional
• Evening dress beneficial to little Malcolm
• Strait-laced class, at Lancing originally
• Academic event: time for the glad rags
• Conforming to standards of correctness
• Conforming to accepted rules or customs
• Not a casual pound for 20 down
• Not a casual quid for Mother
• It's not so casual for Mother to get to the half-century
• Bench taking a line that's rather stiff