The aristocracy are regularly well-birthed (5)
I believe the answer is:
elite
'the aristocracy' is the definition.
(thesaurus)
'regularly well-birthed' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'well'+'birthed'='wellbirthed'
The alternating letters of 'wellbirthed' are 'ELITE'.
'are' is the link.
(Other definitions for elite that I've seen before include "Selected as the best" , "A particular typesize" , "type in pool?" , "Letter size; superior group" , "Creme de la creme" .)