Produced by crossbreeding (6)
I believe the answer is:
hybrid
'produced by crossbreeding' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hybrid that I've seen before include "A composite of mixed origin" , "Plant that is the offspring of two different species" , "Of mixed character" , "Produced by crossbreeding" , "Offspring of mixed parents" .)