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 justify this - if you can you should give a lot more credence to this answer).
'needed' means one lot of letters go next to another.
'a bit of' suggests taking the first letters.
The first 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" .)