The most common and most likely answer to this crossword clue is the 6 letter word TURTLE
I've seen this crossword clue in The Guardian
T
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T
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More clues leading to the result TURTLE
- Time to race along — hard going for distance swimmer
- Time to shoot head off reptile
- One shelled by time bomb releasing hydrogen
- Career time for hot shell-suited swimmer
- Soup? Half of what it’s in, left simmering, female ignored
- Marine creature‘s time to move rapidly without hesitation at first
- The northern dash from East End to find marine animal
- Robot Turing, half cut, let run amok
- This swimmer FBI’s clobbered with rifle-butt?
- Reptile turning over in the soup
Buddy explains!

The straight part
We know that "shell-suited swimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shell-suited swimmer = turtle.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 TURTLE mean?
TURTLE noun- Any land or marine reptile of the order Testudines, characterised by a protective shell enclosing its body. See also tortoise.
- (specifically) A marine reptile of that order.
- An Ancient Roman attack method, where the shields held by the soldiers hide them, not only left, right, front and back, but also from above.
- A type of robot having a domed case (and so resembling the reptile), used in education, especially for making line drawings by means of a computer program.
- An on-screen cursor that serves the same function as a turtle for drawing.
- The curved plate in which the form is held in a type-revolving cylinder press.
- A small element towards the end of a list of items to be bubble sorted, and thus tending to take a long time to be swapped into its correct position. Compare rabbit.
- A breakdancing move consisting of a float during which the dancer's weight shifts from one hand to the other, producing rotation or a circular "walk".
- A low stand for a lamp etc.
- To flip over onto the back or top; to turn upside down.
- To turn and swim upside down.
- To hunt turtles, especially in the water.
- To build up a large defense force and strike only punctually, rather than going for an offensive strategy.
- A turtle dove.