Old oath, say: a “damn” originally? (4)
I believe the answer is:
egad
'old oath' is the definition.
(I've seen this before)
'say a damn originally?' is the wordplay.
'say' becomes 'eg'.
'originally?' indicates taking the first letters (I've seen 'originally' mean this).
The first letter of 'damn' is 'd'.
'eg'+'a'+'d'='EGAD'
(Other definitions for egad that I've seen before include "Archaic exclamation of surprise" , "Oath sworn" , "Mild oath - aged (anag)" , "Archaic expression of anger, surprise, etc" , "Christ" .)