Dismissing one airman's plan (4)
I believe the answer is:
plot
'plan' is the definition.
(plot is a kind of plan)
'dismissing one airman's' is the wordplay.
'dismissing' indicates named letters should be taken away.
'one' becomes 'I' (Roman numeral).
'airman' becomes 'pilot' (pilot is a kind of airman).
'pilot' with 'i' taken away is 'PLOT'.
(Other definitions for plot that I've seen before include "Secret scheme or plan" , "The story told in a novel" , "Storyline or piece of land" , "Connive" , "lost in poor judgment" .)