Part of USAF
I believe the answer is:
force
'part of usaf' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for force that I've seen before include "Compulsion - police" , "Powerful effect or influence" , "Strength - compulsion" , "Compel, oblige" , "Coercion; influence" .)