American taking a long time in customs (6)
I believe the answer is:
'customs' is the definition.
('usage' can be a synonym of 'custom')
'american taking a long time' is the wordplay.
'american' becomes 'US' (United States).
'taking' means one lot of letters go next to another.
'a long time' becomes 'ages' (ages can mean a very long time).
'in' acts as a link.
(Other definitions for usages that I've seen before include "Habitual practices with words" , "Customary treatments, practices" , "Customs." , "speech patterns" , "Linguistic norms" .)