Hobnob (6)
(Other definitions for mingle that I've seen before include "Mix among others" , "Dawn song, poem" , "Join the crowd, mix in" , "Mix together" , "Mix (socially)" .)
'hobnob' is the definition.
This is all the clue.
(Other definitions for fraternise that I've seen before include "Consort" , "sociable" , "Meet socially" , "Be friendly with" , "Trains free (anag.)" .)