Seizure rarest treated (6)
I believe the answer is:
arrest
'seizure' is the definition.
(I know that arrest is a type of seizure)
'rarest treated' is the wordplay.
'treated' indicates an anagram.
'rarest' is an anagram of 'ARREST'.
(Other definitions for arrest that I've seen before include "Land next to the sea" , "Nab" , "Cause to stop" , "Bring to a standstill" , "apprehend a law-breaker" .)