Dirac unfortunately very bitter (5)
I believe the answer is:
acrid
'very bitter' is the definition.
'acrid' can be an answer for 'bitter' (I've seen this before). I'm not sure about the 'very' bit.
'dirac unfortunately' is the wordplay.
'unfortunately' indicates anagramming the letters.
'dirac' anagrammed gives 'ACRID'.
Can you help me to learn more?
(Other definitions for acrid that I've seen before include "Pungent or sharp, like smoke" , "Strong and sharp" , "Bitter - caustic" , "Astringent" , "Greek goddess" .)