Liquor — almost overindulge when upset (4)
I believe the answer is:
grog
'liquor' is the definition.
(I know that grog is a type of rum)
'almost overindulge when upset' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'overindulge' becomes 'gorge' (synonyms).
'when upset' shows that the letters should be reversed in order (in a down clue - set, written, up).
'gorge' with its final letter removed is 'gorg'.
'gorg' in reverse letter order is 'GROG'.
(Other definitions for grog that I've seen before include "Old sailors' tipple" , "Naval rum issue" , "Sailors' drink of rum and water" , "Nautical drink" , "Rum mixed with water" .)