Helpful maxim, not in favour of mood changer (9,4)
I believe the answer is:
auxiliary verb
'changer' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for auxiliary verb that I've seen before include "To do, say" , "(In grammar) a word such as "will"" , "Predicative word defining the mood or tense of another" , "Be, do or have, for example, grammatically -- I buy Rex a rival (anag)" , "Part of speech such as "will" in "he will go"" .)