Married couples regularly RSVP to functions (7)
I believe the answer is:
spouses
'married couples' is the definition.
(I've seen this in another clue)
'regularly rsvp to functions' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'functions' becomes 'uses' ('use' can be a synonym of 'function').
'rsvp'+'to'='rsvpto'
The alternate letters of 'rsvpto' are 'spo'.
'spo'+'uses'='SPOUSES'
(Other definitions for spouses that I've seen before include "Partners in marriage" , "Wives or husbands" .)