Prisoner cleans vet out, getting better (12)
I believe the answer is:
convalescent
'getting better' is the definition.
(I've seen this before)
'prisoner cleans vet out' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'out' is an anagram indicator (out can mean wrong or inaccurate).
'cleans'+'vet'='cleansvet'
'cleansvet' is an anagram of 'valescent'.
'con'+'valescent'='CONVALESCENT'
(Other definitions for convalescent that I've seen before include "getting back to normal" , "One recovering from medical treatment" , "Person picking up" , "One may be showing improvement" , "(Person) getting better" .)