Seize boat that’s docked in reverse (4)
I believe the answer is:
grab
'seize' is the definition.
(grabbing is a kind of seizing)
'boat that's docked in reverse' is the wordplay.
'boat' becomes 'barge' (barge is a kind of boat).
'that's docked' means to remove the last letter (to dock can mean to remove the tail of an animal).
'in reverse' says the letters should be written backwards.
'barge' with its last letter taken off is 'barg'.
'barg' written backwards gives 'GRAB'.
(Other definitions for grab that I've seen before include "Catch hold (of)" , "Seize suddenly" , "Brag about seizure" , "Take quickly" , "Grasp roughly" .)