Curtail quiet tenor's performance (7)
I believe the answer is:
shorten
'curtail' is the definition.
(thesaurus)
'quiet tenor's performance' is the wordplay.
'quiet' becomes 'sh'.
'performance' indicates an anagram.
'tenor' is an anagram of 'orten'.
'sh'+'orten'='SHORTEN'
(Other definitions for shorten that I've seen before include "Put extra fat" , "contract" , "Abbreviate" , "Reduce in length" , "Cut back" .)