Little old colonial outpost making mistake? (8)
I believe the answer is:
Here is my best explanation:
'mistake?' is the definition.
(omission is a kind of mistake)
'little old colonial outpost' is the subsidiary indication.
'little' says to take the initial letters.
'colonial outpost' becomes 'mission'.
The initial letter of 'old' is 'o'.
'making' is the link.
(Other definitions for omission that I've seen before include "failure", "lapse", "Something left out or neglected", "Something left out or forgotten", "Something left out or not done", "Thing left out", "exclusion", "Oversight, piece of work neglected", "Lack" and "Act of leaving out".)