Aggressive kind of dog upset a cousin (10)
I believe the answer is:
pugnacious
'aggressive' is the definition.
(similar in meaning)
'kind of dog upset a cousin' is the wordplay.
'kind of dog' becomes 'pug'.
'upset' indicates anagramming the letters.
'a'+'cousin'='acousin'
'acousin' with letters rearranged gives 'nacious'.
'pug'+'nacious'='PUGNACIOUS'
(Other definitions for pugnacious that I've seen before include "Aggressive" , "Combative" , "Quarrelsome" .)