Appropriate greeting to sailor (6)
I believe the answer is:
hijack
'appropriate' is the definition.
(I've seen this before)
'greeting to sailor' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'to sailor' becomes 'jack' (informal term for a sailor).
'hi'+'jack'='HIJACK'
(Other definitions for hijack that I've seen before include "Seizure of vehicle in motion" , "Rainwater conduit" , "Seize control of an aircraft or ship" , "Illegally divert an aeroplane" , "Take over plane by force in flight" .)