Broadcasting nothing before terrible rain (2,3)
I believe the answer is:
on air
'broadcasting' is the definition.
(broadcasting on the air)
'nothing before terrible rain' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'before' is a charade indicator (letters next to each other).
'terrible' indicates anagramming the letters.
'rain' anagrammed gives 'nair'.
'o'+'nair'='ON-AIR'
(Other definitions for on air that I've seen before include "In the process of broadcasting on radio or TV" , "Broadcasting studio sign" , "Cushioned" , "very happily?" , "Walk in delight here" .)