Mix socially (9)
(Other definitions for associate that I've seen before include "Get together with" , "Make a logical or casual connection" , "Not having full status" , "Join" , "Connect; partner" .)
'mix socially' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hobnob that I've seen before include "Associate on familiar terms" , "Associate familiarly" , "make light conversation" , "Hang out with people of standing" , "Spend time together, socialise" .)