Listened in on radio - drove excessively? (9)
I believe the answer is:
overheard
'listened in' is the definition.
'on radio drove excessively?' is the wordplay.
'on' becomes 'over' (I've seen this in other clues).
'radio drove excessively?' becomes 'heard' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over'+'heard'='OVERHEARD'
(Other definitions for overheard that I've seen before include "Eavesdropped on (a conversation)" , "Accidentally caught" , "Head rover (anag) - listened to secretly" .)