Brave, working with European mammal (6)
I believe the answer is:
beaver
'mammal' is the definition.
(beaver is a kind of mammal)
'brave working with european' is the wordplay.
'working with' indicates anagramming the letters (working with the letters).
'european' becomes 'E' (abbreviation e.g. EU).
'brave' with its letters anagrammed with 'e' can make 'BEAVER'.
(Other definitions for beaver that I've seen before include "Animal with brown fur, builds dams" , "Creature that lives in lodge" , "Facial protection" , "Lodge builder" , "Dam-building rodent" .)