Corrected nouns with one complete agreement (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(I've seen this before)
'corrected nouns with one' is the wordplay.
'corrected' indicates an anagram.
'with' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'nouns' is an anagram of 'unson'.
'unson' going around 'i' is 'UNISON'.
(Other definitions for unison that I've seen before include "Concord, sounding or singing together" , "Accord" , "Total agreement or concord" , "Sameness in pitch" , "Unscrupulous dealer" .)