Agreement - noun is used (6)
I believe the answer is:
unison
'agreement' is the definition.
(I know that unison is a type of agreement)
'noun is used' is the wordplay.
'used' indicates anagramming the letters.
'noun'+'is'='nounis'
'nounis' anagrammed gives 'UNISON'.
(Other definitions for unison that I've seen before include "Musical agreement" , "Unscrupulous dealer" , "The concordant trade union?" , "Unanimity" , "Simultaneous action" .)