It's the custom to avail oneself of silver inside (5)
I believe the answer is:
usage
'it's the custom' is the definition.
(synonyms)
'avail oneself of silver inside' is the wordplay.
'avail oneself of' becomes 'use' (I've seen this before).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'inside' indicates putting letters inside.
'use' going around 'ag' is 'USAGE'.
'to' is the link.
(Other definitions for usage that I've seen before include "Custom - treatment" , "Treatment, practice" , "Way of employing" , "Convention" , "linguistic custom?" .)