Busy, possibly on radio (6)
I believe the answer is:
active
'busy' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for active that I've seen before include "Not passive" , "Engaged, not idle" , "Vigorously moving" , "Moving frequently" , "Keeping busy or operative" .)