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