Another broadcast occupied by one broadcasting (2,3,3)
I believe the answer is:
on the air
'broadcasting' is the definition.
(I've seen this in another clue)
'another broadcast occupied by one' is the wordplay.
'broadcast' indicates an anagram (letters scattered or cast around).
'occupied by' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'one' becomes 'i' (Roman numeral).
'another' anagrammed gives 'onthear'.
'onthear' placed around 'i' is 'ON THE AIR'.
(Other definitions for on the air that I've seen before include "Anti-hero engaged in live broadcast" , "Broadcasting (hence audible)" , "In the process of broadcasting, radio or TV" , "radio listeners can now hear?" .)