Sly leader isn't a good person (5)
I believe the answer is:
saint
'a good person' is the definition.
(I know that saint is a type of good person)
'sly leader isn't' is the wordplay.
'leader' indicates taking the first letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The first letter of 'sly' is 's'.
's'+'aint'='SAINT'
(Other definitions for saint that I've seen before include "Beatified person" , "Holy man" , "Very virtuous person" , "Canonised person" , "Good woman" .)