Thrash metal duo soften (8)
I believe the answer is:
modulate
'soften' is the definition.
(thesaurus)
'thrash metal duo' is the wordplay.
'thrash' is an anagram indicator (I've seen this in other clues).
'metal'+'duo'='metalduo'
'metalduo' with letters rearranged gives 'MODULATE'.
(Other definitions for modulate that I've seen before include "Change tone, pitch etc" , "adjust the volume" , "Regulate" , "Attune" , "Adjust tone or pitch" .)