Outstanding old air force formation (5)
I believe the answer is:
owing
'outstanding' is the definition.
(an outstanding bill is still owing)
'old air force formation' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is present in the answer.
'force' could be 'wing' (wing is a kind of force) and 'wing' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for owing that I've seen before include "Undischarged" , "(Of money) outstanding" , "Unsettled; on account" , "Due to, like debt" , "Outstanding as a debt" .)