Kidnap the sailor on the canal (6)
I believe the answer is:
abduct
'kidnap' is the definition.
('abduct' can be a synonym of 'kidnap')
'the sailor on the canal' is the wordplay.
'the sailor' becomes 'ab' (abbreviation for able seaman).
'on' says to put letters next to each other.
'the canal' becomes 'duct' ('duct' can be a synonym of 'canal').
'ab'+'duct'='ABDUCT'
(Other definitions for abduct that I've seen before include "Take (someone) away by force" , "Kidnap, shanghai" , "Criminally remove to an undisclosed location" , "Take (someone) illegally" , "Carry off illegally" .)