Cross and upset by horse getting shot (6)
I believe the answer is:
hybrid
'cross' is the definition.
(thesaurus)
'upset by horse getting shot' is the wordplay.
'upset' says the letters should be written in reverse.
'horse' becomes 'h' ('H' can be a synonym of 'horse').
'getting' says to put letters next to each other.
'shot' becomes 'rid' (**).
'by'+'h'='byh'
'byh' written backwards gives 'hyb'.
'hyb'+'rid'='HYBRID'
'and' acts as a link.
(Other definitions for hybrid that I've seen before include "Produced by crossbreeding" , "Plant bred from two varieties" , "outcome of miscegenation?" , "Of mixed ancestry" , "Of mixed character" .)