Criticise unopened dumping ground (6)
I believe the answer is:
impugn
'criticise' is the definition.
(thesaurus)
'unopened dumping ground' is the wordplay.
'unopened' means to remove the first letter.
'ground' indicates anagramming the letters (grind the letters ).
'dumping' with its initial letter taken away is 'umping'.
'umping' with letters rearranged gives 'IMPUGN'.
(Other definitions for impugn that I've seen before include "Dispute honesty of (someone's motive)" , "Attack verbally" , "Try to discredit, express doubts about honesty" , "Call into question or attack by words" , "Call into question; criticise" .)