Prevent state heading for totalitarianism (5)
I believe the answer is:
avert
'prevent' is the definition.
(averting is a kind of preventing)
'state heading for totalitarianism' is the wordplay.
'state' becomes 'aver' (both can mean to declare).
'heading for' indicates taking the first letters.
The first letter of 'totalitarianism' is 't'.
'aver'+'t'='AVERT'
(Other definitions for avert that I've seen before include "Ward off (bad event)" , "Ward off, deflect" , "Prevent from happening" , "Stop" , "use preventive measures" .)