Old oath used in England, oddly (4)
I believe the answer is:
egad
'old oath' is the definition.
(I've seen this before)
'england oddly' is the wordplay.
'oddly' indicates alternate letters.
The alternate letters of 'england' are 'EGAD'.
'used in' acts as a link.
(Other definitions for egad that I've seen before include "Archaic expression of anger, surprise, etc" , "My word once" , "Mild oath (archaic)" , "Mild oath - aged (anag)" , "Christ" .)