Society records over 50 regular constituents (7)
I believe the answer is:
staples
'regular constituents' is the definition.
(staple can mean a frequent participant or feature)
'society records over 50' is the wordplay.
'society' becomes 's' (common abbreviation in names of organisations).
'records' becomes 'tapes' ('tape' can be a synonym of 'record'**).
'over' indicates putting letters inside.
's0' becomes 'l' (roman numeral).
'tapes' going around 'l' is 'taples'.
's'+'taples'='STAPLES'
(Other definitions for staples that I've seen before include "Essential items" , "Paper fasteners" , "Get them from a stationer" , "most important items!" , "Wire fastenings" .)