Old-timey "Holy moly!" (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
egad
(Other definitions for egad that I've seen before include "Oath sworn" , "Archaic expression of anger, surprise, etc" , "Old exclamation; aged (anagram)" , "Mild oath - aged (anag)" , "traditional oath" .)