The bitterness we all have (6)
I believe the answer is:
spleen
'the bitterness' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for spleen that I've seen before include "Vent it to show anger" , "Bodily organ associated with bad humour" , "Internal organ - irascibility" , "Acrimony" , "Organ associated with ill humour" .)