Thwarted by interbreeding? (7)
I believe the answer is:
crossed
'thwarted by interbreeding?' is the definition.
'crossed' can be an answer for 'thwarted' ('cross' can be a synonym of 'thwart'). I am not certain of the 'by interbreeding?' bit.
'thwarted by interbreeding?' is the wordplay.
'thwarted' becomes 'ed' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'interbreeding?' becomes 'cross' ('cross' can be a synonym of 'interbreeding').
'ed' after 'cross' is 'CROSSED'.
Can you help me to learn more?
(Other definitions for crossed that I've seen before include "Came over and < annoyed" , "Passed" , "Went over to the other side" , "Traversed" , "Intersected; opposed" .)