In the past, when an air gunner needed a bit of oxygen (3)

I believe the answer is:

ago

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'in the past' is the definition.
(synonyms)

'an air gunner needed a bit of oxygen' is the wordplay.
'an' becomes 'a'.
'air gunner' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'needed' is a charade indicator (letters next to each other).
'a bit of' says to take the initial letters.
The first letter of 'oxygen' is 'o'.
'a'+'g'+'o'='AGO'

'when' is the link.

Can you help me to learn more?

(Other definitions for ago that I've seen before include "Previously" , "In past times" , "Back (in time)" , "Earlier, gone by" , "Gone, past, since" .)

I've seen this clue in the Evening Standard.

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