A certain bitterness is good for everybody (4)
I believe the answer is:
gall
'a certain bitterness' is the definition.
'gall' can be an answer for 'bitterness' ('gall' can be a synonym of 'bitterness'). I'm unsure of the 'a certain' bit.
'good for everybody' is the wordplay.
'good for' becomes 'g' (I've seen this in other clues).
'everybody' becomes 'all' (I've seen this before).
'g'+'all'='GALL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for gall that I've seen before include "Audacity" , "Impudence, cheek" , "Impudence - annoy" , "Brass nedk or bile" , "A sore spot or a bodily bladder" .)