European in Chad mistreated mammal (7)
I believe the answer is:
echidna
'mammal' is the definition.
(echidna is a kind of mammal)
'european in chad mistreated' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'mistreated' is an anagram indicator.
'in'+'chad'='inchad'
'inchad' anagrammed gives 'chidna'.
'e'+'chidna'='ECHIDNA'
(Other definitions for echidna that I've seen before include "Creature with spines" , "Prickly creature" , "Toothless type" , "Insectivorous mammal" , "One preying on the workers" .)