On air, encourage understanding (7)
I believe the answer is:
insight
'understanding' is the definition.
(I know that insight is a type of understanding)
'on air encourage' is the wordplay.
'on air' indicates a 'sounds like' (homophone) clue (how the word sounds on the radio, on air).
'encourage' becomes 'incite' (I've seen this before).
'incite' sounds like 'INSIGHT'.
(Other definitions for insight that I've seen before include "Sudden understanding" , "percipience" , "Awareness" , "apprehension" , "Perspicacity" .)