About to master four-letter word usages (7)
I believe the answer is:
customs
'usages' is the definition.
('custom' can be a synonym of 'usage')
'about to master four-letter word' is the wordplay.
'about' is an insertion indicator.
'to master' becomes 'm'.
'four letter word' becomes 'custos' (I can't explain this - if you can you should give a lot more credence to this answer).
'm' inserted into 'custos' is 'CUSTOMS'.
Can you help me to learn more?
(Other definitions for customs that I've seen before include "state's duties?" , "Port/airport department" , "Accepted practices" , "Traditions" , "Import duties" .)