Familiarity (5)
'familiarity' is the definition.
(synonyms)
This is all the clue.
(Other definitions for acquaintance that I've seen before include "contact" , "Person known, but not a close friend" , "Familiarity" , "Person known slightly" , "Slight knowledge" .)
(Other definitions for habit that I've seen before include "It's worn in order" , "Propensity" , "Routine - clothing" , "Ingrained behaviour" , "addictive practice" .)