Possible Clues:

• Helpful maxim, not in favour of mood changer
• Have to be seen as one in the grammar class?
• Be, do or have, for example, grammatically — I buy Rex a rival (anag)
• In modifying part of speech, I left in gold cross surmounting intricate breviary
• Perhaps has-been shows more than one side, dishonest one, in bustling Avebury
• Part of speech that combines with another of the same kind
• Can, for example
• (In grammar) a word such as 'will'
• Part of speech such as 'will' in 'he will go'
• To do, say, Rievaulx by air is unusual