Hardened animal doctor stops trainee misbehaving (10)
I believe the answer is:
inveterate
'hardened' is the definition.
(I've seen this before)
'animal doctor stops trainee misbehaving' is the wordplay.
'animal doctor' becomes 'vet' (veterinarian).
'stops' indicates putting letters inside.
'misbehaving' indicates anagramming the letters.
'trainee' anagrammed gives 'inerate'.
'vet' going into 'inerate' is 'INVETERATE'.
(Other definitions for inveterate that I've seen before include "Confirmed in (bad) habit" , "Firmly established by usage or custom" , "Habitual" , "long-standing" , "Long-established" .)