Cross and upset by horse getting shot (6)
I believe the answer is:
Here is my best explanation:
'cross' is the definition.
(I've seen this before)
'upset by horse getting shot' is the subsidiary indication.
'upset' is a reversal indicator.
'horse' becomes 'h' ('H' can be a synonym of 'horse').
'getting' says to put letters next to each other.
'shot' becomes 'rid' (**).
'byh' reversed gives 'hyb'.
'and' is the link.
(Other definitions for hybrid that I've seen before include "Cross of two different species or varieties", "mix", "(One) of mixed origin", "outcome of miscegenation?", "Offspring of two different species or varieties", "Composite of mixed origin", "Of mixed ancestry", "Crossbreed or mixture", "Offspring of mixed parents" and "Plant grown from two different species".)