Vessel in question south of Cuba, originally (4)
I believe the answer is:
cask
'vessel' is the definition.
(cask is a kind of vessel)
'question south of cuba originally' is the wordplay.
'question' becomes 'ask' (asking a question).
'south of' means one lot of letters go next to another (in a down clue, some letters go under others).
'originally' indicates taking the first letters.
The initial letter of 'cuba' is 'c'.
'ask' put after 'c' is 'CASK'.
'in' is the link.
(Other definitions for cask that I've seen before include "Large barrel" , "Measure" , "Cylinder for liquid" , "Wine vessel; sack (anag.)" , "Liquor container" .)