Something bitter that's good for everybody (4)
I believe the answer is:
'something bitter' is the definition.
(gall can mean bile, a bitter substance)
'good for everybody' is the wordplay.
'good' becomes 'g' (abbreviation).
'for' is a charade indicator (letters next to each other).
'everybody' becomes 'all' (I've seen this in another clue).
'that's' is the link.
(Other definitions for gall that I've seen before include "Ill-will - bile" , "Impudence - annoy" , "Effrontery" , "Audacity" , "Liver's secretion" .)