Prime Minister on the radio? Blast! (5)
I believe the answer is:
blare
'blast' is the definition.
(I know that blast can be written as blare)
'prime minister on the radio?' is the wordplay.
'prime minister' becomes 'Blair' (Tony Blair, former UK prime minister).
'on the radio?' indicates a 'sounds like' (homophone) clue (I've seen 'on the radio' mean this).
'blair' is a homophone of 'BLARE'.
(Other definitions for blare that I've seen before include "Sound loudly" , "Roar" , "Loud harsh noise" , "Strident sound" , "Sound loud and harsh" .)