Seizes sailor's round vessel (7)
I believe the answer is:
abducts
'seizes' is the definition.
(abducting is a kind of seizing)
'sailor's round vessel' is the wordplay.
'sailor's' becomes 'abs' (abbreviation for able seaman).
'round' means one lot of letters goes inside another.
'vessel' becomes 'duct'.
'abs' going around 'duct' is 'ABDUCTS'.
(Other definitions for abducts that I've seen before include "Kidnaps" , "Takes away" , "Makes off with" , "Takes by force" .)