Seizure rarest possibly (6)
I believe the answer is:
arrest
'seizure' is the definition.
(arrest is a kind of seizure)
'rarest possibly' is the wordplay.
'possibly' is an anagram indicator.
'rarest' with letters rearranged gives 'ARREST'.
(Other definitions for arrest that I've seen before include "Stop (on suspicion)" , "Bring to a standstill" , "Sartre (anag) -- check" , "Take into legal custody" , "Apprehend; detain" .)