For example, commercial oath once (4)

I believe the answer is:

egad

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'oath once' is the definition.
I can't tell whether this definition defines the answer.

'for example commercial' is the wordplay.
'for example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'commercial' becomes 'ad' (abbreviation for advertisement).
'eg'+'ad'='EGAD'

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(Other definitions for egad that I've seen before include "Oath sworn" , "Archaic exclamation of surprise" , "Archaic expression of anger, surprise, etc" , "Mild oath (archaic)" , "My word once" .)

I've seen this clue in The Times.

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