In the past, when an air gunner needed a bit of oxygen (3)
I believe the answer is:
ago
'in the past' is the definition.
(three years ago means three years in the past)
'an air gunner needed a bit of oxygen' is the wordplay.
'an' becomes 'a'.
'air gunner' becomes 'g' (I can't explain this - if you can 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 initial letter of 'oxygen' is 'o'.
'a'+'g'+'o'='AGO'
'when' acts as a 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" .)