Rarest form of seizure (6)
I believe the answer is:
arrest
'seizure' is the definition.
(I know that arrest is a type of seizure)
'rarest form' is the wordplay.
'form' indicates an anagram.
'rarest' anagrammed gives 'ARREST'.
'of' is the link.
(Other definitions for arrest that I've seen before include "Run in" , "Sartre (anag) -- check" , "Land next to the sea" , "Cause to stop" , "Stop - take into custody" .)