Animal backbone? Not good guess (10)
I believe the answer is:
assumption
'guess' is the definition.
(thesaurus)
'animal backbone? not good' is the wordplay.
'animal' becomes 'ass' (term for a donkey).
'backbone?' becomes 'gumption' (gumption can mean spirit or backbone).
'not' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gumption' with 'g' taken out is 'umption'.
'ass'+'umption'='ASSUMPTION'
(Other definitions for assumption that I've seen before include "Church festival" , "Catholic feast" , "Belief" , "Supposition" , "taken for granted" .)