Saw retreats, letting out a mild oath (4)
I believe the answer is:
egad
'mild oath' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for egad that I've seen before include "My word once" , "traditional oath" , "Old exclamation; aged (anagram)" , "Forgotten curse" , "Christ" .)