Crossword Genius

In a world atlas, three ____ are Bering, Palk and Sunda (7)

Ross

I believe the answer is:

straits

I'm a little stuck... Click here to teach me more about this clue!

'in' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'a world atlas three are bering palk and sunda' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'three' could be 'tri' (prefix meaning 'three') and 'tri' is found in the remaining letters.
The remaining letters 'sts' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for straits that I've seen before include "Sea passages" , "Difficulties" , "Dire --, rock band" , "... of Magellan, Hormuz" , "Narrow channels of the sea joining two larger bodies of water" .)

I've seen this clue in The Times.
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