Head of state is not, in short, a good person (5)
I believe the answer is:
saint
'a good person' is the definition.
(saint is a kind of good person)
'head of state is not' is the wordplay.
'head of' indicates taking the first letters.
'is not' becomes 'ain't' ('ain't' is a non-standard form of 'isn't').
The first letter of 'state' is 's'.
's'+'aint'='SAINT'
'in short' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for saint that I've seen before include "Scotland's oldest university" , "Holy person" , "eg John, Paul or George" , "Good woman" , "Holy man" .)