Kidnap sailor on tube (6)
I believe the answer is:
abduct
'kidnap' is the definition.
(I know that kidnap can be written as abduct)
'sailor on tube' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'tube' becomes 'duct' (type of tube or canal).
'ab'+'duct'='ABDUCT'
(Other definitions for abduct that I've seen before include "Hijack" , "Kidnap, carry person off by force" , "Kidnap, shanghai" , "Criminally remove to an undisclosed location" , "Take (someone) away by force" .)