The ghosts of grog (7)
I believe the answer is:
spirits
I believe this is a double definition.
'the ghosts' is the first definition.
(I've seen this in another clue)
'grog' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for spirits that I've seen before include "Gin, vodka, rum etc" , "Ghosts" , "Animating forces or hard liquors" , "eg Gin, whisky" , "Hard liquor" .)