Politician prior to mistakes starting? That is right (7)
I believe the answer is:
premier
'politician' is the definition.
(I know that premier is a type of head of state)
'prior to mistakes starting? that is right' is the wordplay.
'prior to' becomes 'pre' (I've seen this in another clue).
'starting?' indicates taking the first letters.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'right' becomes 'r' (common abbreviation).
The first letter of 'mistakes' is 'm'.
'pre'+'m'+'ie'+'r'='PREMIER'
(Other definitions for premier that I've seen before include "First - head of government" , "First in rank or degree" , "Foremost" , "Which is having precedence" , "Earliest" .)