Custom in American era (5)
I believe the answer is:
usage
'custom' is the definition.
(I know this)
'american era' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'era' becomes 'age' (both can mean a long period of time).
'us'+'age'='USAGE'
'in' acts as a link.
(Other definitions for usage that I've seen before include "Tradition - rate of consumption" , "Common practice" , "etiquette" , "Custom - treatment" , "Usual practice" .)