Official survey held by Saracens usually (6)
I believe the answer is:
census
'official survey' is the definition.
(I've seen this before)
'held by saracens usually' is the wordplay.
'held by' indicates a hidden word.
'CENSUS' is hidden within 'saracens usually'.
(Other definitions for census that I've seen before include "Periodical population count" , "Periodic counting of the population" , "Official survey" , "Official body count?" , "Periodic count by statisticians" .)