Employ silver, in practice (5)
I believe the answer is:
usage
'employ' is the definition.
(thesaurus)
'silver in practice' is the wordplay.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'in' is an insertion indicator.
'practice' becomes 'use' (use is a kind of practice).
'ag' going within 'use' is 'USAGE'.
(Other definitions for usage that I've seen before include "Tradition - rate of consumption" , "Style of language" , "Terminology - custom" , "Habit" , "Employment - practice" .)