Acerbic comeback suppressed by adjudicator (4)
I believe the answer is:
acid
'acerbic' is the definition.
(synonyms)
'comeback suppressed by adjudicator' is the wordplay.
'comeback' is a reversal indicator.
'suppressed by' indicates a hidden word.
'dica' is hidden in the letters of 'adjudicator'.
'dica' backwards is 'ACID'.
(Other definitions for acid that I've seen before include "Sour-tasting" , "Bitter substance" , "Type of chemical" , "Sour - type of rain or drop" , "Substance with pH of less than 7" .)