The more prevalent blood group these days (6,3)
I believe the answer is:
common era
'these days' is the definition.
(the current era)
'the more prevalent blood group' is the wordplay.
'the more prevalent' becomes 'commoner' (more common).
'blood group' becomes 'A' (A is one of the main blood groups).
'commoner'+'a'='COMMON ERA'
(Other definitions for common era that I've seen before include "this time" , "In Christian times" , "these days" .)