Possible Clues:

• School subject
• It's as liable to add it up or take it away
• It concerns numbers, in short
• Looking back, charlatan keeps time - it's one of those things one learns in school
• Arithmetic, to Brits
• One of the subjects reviews The Imposter outside the capital
• The science of number, quantity and space
• British subject?
• After before them follow them in numbers of ways
• Some study, by the sound of it