Diluted rum (4)
I believe the answer is:
grog
'diluted rum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for grog that I've seen before include "Mixture of spirit and water" , "Spirits and water mixture" , "Old naval drink" , "Drink of spirits mixed with water" , "Rum cut with water" .)