The most common and most likely answer to this crossword clue is the 10 letter word BACKSTROKE
I've seen this crossword clue in Independent
B
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Buddy explains!
The straight part
We know that "movement a swimmer might start" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case movement a swimmer might start = backstroke.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 BACKSTROKE mean?
BACKSTROKE noun- A swimming stroke swum lying on one's back, while rotating both arms through the water as to propel the swimmer backwards.
- (bellringing) The pull on the tail of the rope that swings the bell through a full circle (compare handstroke)
- To swim the backstroke.