Seize for ransom (6)
I believe the answer is:
kidnap
'seize for ransom' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for kidnap that I've seen before include "Take person prisoner for ransom" , "deprive it of liberty" , "Seize and hold for ransom" , "Seize, usually for ransom" , "'Abduct, snatch (6)'" .)