Fighting spirit of dog can upset it over years (9)
I believe the answer is:
pugnacity
'fighting spirit' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe there's an association between them I don't understand?
'dog can upset it over years' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'upset' is a reversal indicator.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'years' becomes 'y' (abbreviation).
'can' backwards is 'nac'.
'pug'+'nac'+'it'+'y'='PUGNACITY'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for pugnacity that I've seen before include "Aggressiveness, eagerness to fight" , "Belligerence" .)