The most common and most likely answer to this crossword clue is the 5 letter word MODAL
M
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D
A
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The straight part
We know that "auxiliary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case auxiliary = modal.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MODAL mean?
MODAL noun- A modal proposition.
- A modal form, notably a modal auxiliary.
- (grammar) A modal verb.
- A modal window, one that cannot be closed until a decision is made.
- Of, or relating to a mode or modus
- (grammar) of, relating to, or describing the mood of a clause
- Of, relating to, or composed in the musical modi by which an octave is divided, associated with emotional moods in Ancient — and in medieval ecclesiastical music
- Of, or relating to the modality between propositions
- Relating to the statistical mode.
- Having separate modes in which user input has different effects.
- Requiring immediate user interaction and thus presented so that it cannot be closed or interacted behind until a decision is made.
- Relating to the form of a thing rather to any of its attributes