Related Clues:

• Reason for difficulty learning to do some mathematics
• Put on halfway through first millennium in Rome?
• Freely include argument swallowed by lightweight
• Tot; append
• Tot up
• Subtract's opposite
• Hit the plus button
• Stir in, say
• Make sense (with "up")
• Do a grade-school lesson
• Join the conversation
• Move to continue playing football, or concede defeat
• Postfix
• Say also
• Calculate, in a way