Watered-down rum (4)
I believe the answer is:
grog
'watered-down rum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for grog that I've seen before include "Rum mixed with water" , "Traditional naval drink" , "Mixture of spirit and water" , "Sailors' drink of rum and water" , "Drink of spirits and water liked by sailors" .)