Head of state 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)
'head of state isn't' is the wordplay.
'head of' indicates taking the first letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The first letter of 'state' is 's'.
's'+'aint'='SAINT'
(Other definitions for saint that I've seen before include "See 6" , "Very virtuous person" , "Scotland's oldest university" , "paragon!" , "eg Pancras" .)