Office for National Statistics breaking big survey agreement (9)
I believe the answer is:
consensus
'agreement' is the definition.
(I know that consensus is a type of agreement)
'office for national statistics breaking big survey' is the wordplay.
'office for national statistics' becomes 'ONS' (abbreviation).
'breaking' is an insertion indicator.
'big survey' becomes 'census' (a census is a survey).
'ons' put inside 'census' is 'CONSENSUS'.
(Other definitions for consensus that I've seen before include "General agreement" , "Majority view" , "agreed" , "Unanimity" , "General opinion" .)