Seize greeting card (6)
I believe the answer is:
hijack
'seize' is the definition.
(hijacking is a kind of seizing)
'greeting card' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'card' becomes 'jack' (type of playing card**).
'hi'+'jack'='HIJACK'
(Other definitions for hijack that I've seen before include "Seize a vehicle forcefully" , "Seize control of vehicle in transit" , "Take control without authority" , "Illegally commandeer and divert" , "Take over plane by force in flight" .)