The most common and most likely answer to this crossword clue is the 9 letter word CHIVVYING
I've seen this crossword clue in The Times
C
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V
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The straight part
We know that "Nagging" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nagging = chivvying.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 CHIVVYING mean?
CHIVVYING verb- To coerce or hurry along, as by persistent request.
- To subject to harassment or verbal abuse.
- To sneak up on or rapidly approach.
- To pursue as in a hunt.
- The act of one who chivvies, or hurries somebody along; an urging.