Crossword Genius

It's found between breasts and rear ends when one goes topless (7)

Ross

I believe the answer is:

sternum

'it's found between breasts' is the definition.
(the sternum is the breastbone)

'rear ends when one goes topless' is the wordplay.
I cannot really see how this works, but
'rear' could be 'stern' (synonyms) and 'stern' is found in the answer.
The remaining letters 'um' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'and' acts as a link.

(Other definitions for sternum that I've seen before include "big bone" , "front of breast" , "Breastbone" , "Chest bone" , "Munster (anag.)" .)

I've seen this clue in The Guardian.
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