Expropriate greeting card? (6)
I believe the answer is:
hijack
'expropriate' is the definition.
(synonyms)
'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 "Illegally commandeer and divert" , "Illegally take over (a plane or ship)" , "Steal, in transit" , "Rainwater conduit" , "Illegal activity" .)