Custom from American era (5)
I believe the answer is:
usage
'custom' is the definition.
(I know that custom can be written as usage)
'american era' is the wordplay.
'american' becomes 'US' (United States).
'era' becomes 'age' (both can mean a long period of time).
'us'+'age'='USAGE'
'from' acts as a link.
(Other definitions for usage that I've seen before include "Utilisation" , "Acceptable language" , "Common practice" , "Custom; practice" , "management" .)