It’s probably hearts she breaks! (7)
I believe the answer is:
smasher
'it's probably hearts' is the definition.
I can't tell whether this defines the answer.
'she breaks' is the wordplay.
I cannot quite understand how this works, but
'she' could be 'sher' and 'sher' is found within the answer.
The remaining letters 'sma' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for smasher that I've seen before include "an attacking tennis player?" , "who is attractive to men?" , "responsible for break-up?" , "Wonderful thing" , "Pretty woman" .)