Slow boat going to Spain, transporter that's sunk (4)
I believe the answer is:
tube
'transporter that's sunk' is the definition.
(underground transport)
'slow boat going to spain' is the wordplay.
'slow boat' becomes 'tub' (I've seen this in another clue).
'going to' says to put letters next to each other.
'spain' becomes 'e' (abbreviation for Espana).
'tub'+'e'='TUBE'
(Other definitions for tube that I've seen before include "Can of beer (colloquial)" , "Pack of toothpaste; type of train" , "London Underground or long cylinder" , "Hollow body for the underground" , "appearing tinny?" .)