Airman losing one plan (4)
I believe the answer is:
plot
'plan' is the definition.
(I know that plan is a is a more specific form of the action of plot)
'airman losing one' is the wordplay.
'airman' becomes 'pilot' (pilot is a kind of airman).
'losing' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'pilot' with 'i' removed is 'PLOT'.
(Other definitions for plot that I've seen before include "Storyline or piece of land" , "A scheme, or a parcel of land" , "See 12 Across" , "Patch of ground" , "Secret scheme to do something underhand" .)