Hello John: this is a robbery (6)
I believe the answer is:
hijack
'a robbery' is the definition.
(I know that hijack is a type of hijacking)
'hello john' is the wordplay.
'hello' becomes 'hi' (synonyms).
'john' becomes 'jack' (I've seen this in another clue).
'hi'+'jack'='HIJACK'
'this is' is the link.
(Other definitions for hijack that I've seen before include "Illegally divert an aeroplane" , "Take (aircraft) by force" , "Illegal activity" , "Illegally take over (a plane or ship)" , "Rainwater conduit" .)