The most common and most likely answer to this crossword clue is the 9 plus 4 letter word AUXILIARY VERB
A
U
X
I
L
I
A
R
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V
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R
B
More clues leading to the result AUXILIARY VERB
Buddy explains!
The straight part
We know that "modifying part of speech" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case modifying part of speech = auxiliary verb.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.