"Herb" confiscated by American customs (6)
I believe the answer is:
usages
'customs' is the definition.
('usage' can be a synonym of 'custom')
'herb confiscated by american' is the wordplay.
'herb' becomes 'sage' (sage is a kind of herb).
'confiscated by' means one lot of letters goes inside another.
'american' becomes 'US' (United States).
'sage' inserted into 'us' is 'USAGES'.
(Other definitions for usages that I've seen before include "Habitual practices with words" , "Linguistic norms" , "Habitual or customary practices" , "Customs." , "Customary treatments, practices" .)