Commandeer a greeting card (6)
I believe the answer is:
hijack
'commandeer' is the definition.
(I know that commandeer can be written as hijack)
'a greeting card' is the wordplay.
'a 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 control of an aircraft or ship" , "Take over forcibly" , "Criminal seizure of plane" , "Take (aircraft) by force" , "Steal en route" .)