Bitter Aussie rockers covering two singles separately (6)
I believe the answer is:
acidic
'bitter' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
(Other definitions for acidic that I've seen before include "Like vinegar" , "Unpleasant" , "Vinegary" , "that leaves a bitter taste" , "Sour; bitter" .)