Custom, practice (5)
I believe the answer is:
usage
I believe this clue is a double definition.
'custom' is the first definition.
(synonyms)
'practice' is the second definition.
(usage is a kind of practice)
(Other definitions for usage that I've seen before include "phraseology" , "Acceptable language" , "linguistic custom?" , "Usual practice" , "Established custom" .)