Prevent state involvement, ultimately (5)
I believe the answer is:
avert
'prevent' is the definition.
(averting is a kind of preventing)
'state involvement ultimately' is the wordplay.
'state' becomes 'aver' (I've seen this before**).
'ultimately' suggests the final letters.
The last letter of 'involvement' is 't'.
'aver'+'t'='AVERT'
(Other definitions for avert that I've seen before include "Prevent from happening" , "Turn aside or ward off" , "Turn away or aside" , "Turn away, prevent" , "Prevent from occurring" .)