The most common and most likely answer to this crossword clue is the 7 letter word NOCKING
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Buddy explains!

The straight part
We know that "Fitting an arrow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fitting an arrow = nocking.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 NOCKING mean?
NOCKING verb- To fit an arrow against the bowstring of a bow or crossbow. (See also notch.)
- To cut a nock in (usually in an arrow's base or the tips of a bow).