Customs of the American Middle-West (4)
I believe the answer is:
uses
'customs' is the definition.
(use is a kind of custom)
'the american middle-west' is the wordplay.
'the american' becomes 'us'.
'middle' says to take the centre.
The centre of 'west' is 'es'.
'us'+'es'='USES'
'of' acts as a link.
(Other definitions for uses that I've seen before include "Customs" , "Consumes; treats" , "manipulates" , "Puts into service" , "Takes (drugs)" .)