Breed of dog, one lean aggressive type (8)
I believe the answer is:
pugilist
'aggressive type' is the definition.
'breed of dog one lean' is the wordplay.
'breed of dog' becomes 'pug'.
'one' becomes 'i' (Roman numeral).
'lean' becomes 'list' (I've seen this before).
'pug'+'i'+'list'='PUGILIST'
(Other definitions for pugilist that I've seen before include "A professional boxer" , "Fist fighter" , "Prize fighter" .)