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

The straight part
We know that "style of swimming" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case style of swimming = trudgen.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 TRUDGEN mean?
TRUDGEN noun- A swimming stroke in which the left and right hands are alternately raised out of the water, brought forward and drawn back through the water. A scissor kick is performed on every second stroke.