Time to follow American custom (5)
I believe the answer is:
usage
'custom' is the definition.
(I know that custom can be written as usage)
'time to follow american' is the wordplay.
'time' becomes 'age' (an age is a period of time).
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'american' becomes 'US' (United States).
'age' after 'us' is 'USAGE'.
(Other definitions for usage that I've seen before include "Silly little thing" , "Treatment; tradition" , "Utilisation" , "Habitual custom" , "Treatment, practice" .)