The most common and most likely answer to this crossword clue is the 3 plus 2 letter word RUN IN
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More clues leading to the result RUN IN
Buddy explains!

The straight part
We know that "Enter at speed/arrest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case enter at speed/arrest = run in.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 RUN IN mean?
RUN IN noun- An encounter; a scrape or brush, especially one involving trouble or difficulty.
- The end-phase of a competition.